This is still somehow a WIP, we have some issues with this interface
that are not trivial to solve. This patch tries to make the concepts of
RegionBranchPoint and RegionSuccessor more robust and aligned with their
definition:
- A `RegionBranchPoint` is either the parent (`RegionBranchOpInterface`)
op or a `RegionBranchTerminatorOpInterface` operation in a nested
region.
- A `RegionSuccessor` is either one of the nested region or the parent
`RegionBranchOpInterface`

Some new methods with reasonnable default implementation are added to
help resolving the flow of values across the RegionBranchOpInterface.

It is still not trivial in the current state to walk the def-use chain
backward with this interface. For example when you have the 3rd block
argument in the entry block of a for-loop, finding the matching operands
requires to know about the hidden loop iterator block argument and where
the iterargs start. The API is designed around forward-tracking of the
chain unfortunately.

Try to reland #161575 ; I suspect a buildbot incremental build issue.

Reverts llvm/llvm-project#161575

Broke Windows on ARM buildbot build, needs investigations.

This is still somehow a WIP, we have some issues with this interface
that are not trivial to solve. This patch tries to make the concepts of
RegionBranchPoint and RegionSuccessor more robust and aligned with their
definition:
- A `RegionBranchPoint` is either the parent (`RegionBranchOpInterface`)
op or a `RegionBranchTerminatorOpInterface` operation in a nested
region.
- A `RegionSuccessor` is either one of the nested region or the parent
`RegionBranchOpInterface`

Some new methods with reasonnable default implementation are added to
help resolving the flow of values across the RegionBranchOpInterface.

It is still not trivial in the current state to walk the def-use chain
backward with this interface. For example when you have the 3rd block
argument in the entry block of a for-loop, finding the matching operands
requires to know about the hidden loop iterator block argument and where
the iterargs start. The API is designed around forward-tracking of the
chain unfortunately.

We are using the symbol table machinery to lookup for a callable, but
when the analysis scope if a function, such lookup will resolve outside
of the scope. This can lead to race-condition issues since other passes
may operate in parallel on the sibling functions.
The callable would be discarded right after the lookup (we check the
analysis scope), so avoiding the lookup is NFC.

For the DataFlow solver, we're looking at the top-level operation, and
if it isn't a SymbolTable we disable the interprocedural optimization in
the solver config directly.
This strategy isn't NFC but seems reasonnable and does not encounter any
change in behavior in practice in tree.

Fix #154948

This patch is forcing all values to be initialized by the
LivenessAnalysis, even in dead blocks. The dataflow framework will skip
visiting values when its already knows that a block is dynamically
unreachable, so this requires specific handling.
Downstream code could consider that the absence of liveness is the same
a "dead".
However as the code is mutated, new value can be introduced, and a
transformation like "RemoveDeadValue" must conservatively consider that
the absence of liveness information meant that we weren't sure if a
value was dead (it could be a newly introduced value.

Fixes #153906

This commit introduces `visitCallOperation` and `visitCallableOperation`
extension points in the sparse data flow analysis framework. This
allows, for example, to make the analysis less conservative, without a
lot of code duplication, propagating information even if not all the
call or return sites are known.

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.

The concept of a 'program point' in the original data flow framework is
ambiguous. It can refer to either an operation or a block itself. This
representation has different interpretations in forward and backward
data-flow analysis. In forward data-flow analysis, the program point of
an operation represents the state after the operation, while in backward
data flow analysis, it represents the state before the operation. When
using forward or backward data-flow analysis, it is crucial to carefully
handle this distinction to ensure correctness.

This patch refactors the definition of program point, unifying the
interpretation of program points in both forward and backward data-flow
analysis.

How to integrate this patch?

For dense forward data-flow analysis and other analysis (except dense
backward data-flow analysis), the program point corresponding to the
original operation can be obtained by `getProgramPointAfter(op)`, and
the program point corresponding to the original block can be obtained by
`getProgramPointBefore(block)`.

For dense backward data-flow analysis, the program point corresponding
to the original operation can be obtained by
`getProgramPointBefore(op)`, and the program point corresponding to the
original block can be obtained by `getProgramPointAfter(block)`.

NOTE: If you need to get the lattice of other data-flow analyses in
dense backward data-flow analysis, you should still use the dense
forward data-flow approach. For example, to get the Executable state of
a block in dense backward data-flow analysis and add the dependency of
the current operation, you should write:

``getOrCreateFor<Executable>(getProgramPointBefore(op),
getProgramPointBefore(block))``

In case above, we use getProgramPointBefore(op) because the analysis we
rely on is dense backward data-flow, and we use
getProgramPointBefore(block) because the lattice we query is the result
of a non-dense backward data flow computation.

related dsscussion:
https://discourse.llvm.org/t/rfc-unify-the-semantics-of-program-points/80671/8
corresponding PSA:
https://discourse.llvm.org/t/psa-program-point-semantics-change/81479

Relands #100361 with fixed dependencies.