In PromoteMem2Reg, we perform a DFS over the CFG and track, for each
alloca, its incoming value and its associated incoming DebugLoc, both of
which are taken from stores to that alloca; these values and DebugLocs
are propagated to PHI nodes when new blocks are reached. In the event
that for one incoming edge no store instruction has been seen, we
propagate an UndefValue and an empty DebugLoc to the PHI.

This is a perfectly valid occurrence, and assigning an empty DebugLoc to
the PHI is the correct course of action; therefore, we should pass an
annotated DebugLoc instead, so that in DebugLoc coverage tracking we
correctly do not expect a valid DebugLoc to be present; we generally
mark allocas as having CompilerGenerated locations, so I've chosen to
use the same annotation to represent the uninitialized value of that
alloca.

This change is NFC outside of DebugLoc coverage tracking builds.

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 140 instances that rely on this "redirection", with the
vast majority of them under llvm/. Since relying on the redirection
doesn't improve readability, this patch replaces SmallSet with
SmallPtrSet for pointer element types.

This is one of the final remaining debug-intrinsic specific codepaths
out there, and pieces of cross-LLVM infrastructure to do with debug
intrinsics.

At this stage I'm just opportunistically deleting any code using
debug-intrinsic types, largely adjacent to calls to findDbgUsers. I'll
get to deleting that in probably one or more two commits.

SROA and a few other facilities use generic-lambdas and some overloaded
functions to deal with both intrinsics and debug-records at the same time.
As part of stripping out intrinsic support, delete a swathe of this code
from things in the Utils directory.

This is a large diff, but is mostly about removing functions that were
duplicated during the migration to debug records. I've taken a few
opportunities to replace comments about "intrinsics" with "records",
and replace generic lambdas with plain lambdas (I believe this makes
it more readable).

All of this is chipping away at intrinsic-specific code until we get to
removing parts of findDbgUsers, which is the final boss -- we can't
remove that until almost everything else is gone.

When BasicBlock has a large number of allocas, and
successors, we had to copy entire IncomingVals and
IncomingLocs vectors for successors.

Also updates to IncomingVals and
IncomingLocs are infrequent (only Load/Store into
alloca affect arrays).

Given the nature of DFS traversal, instead of copying
the entire vector, we can keep track of the changes
and undo all changes done by successors.

Fixes #142461

On the attached to issue #142461 IR RSS drops from 35Gb to 1.8Gb.

But it does not affect compile time on average

https://llvm-compile-time-tracker.com/compare.php?from=2e98ed8caa0b47ee79af4ad24b5436a89fe49dfa&to=effac6d1fd600e544f8bc21382c7e541973b1378&stat=instructions:u

They are all DFS state related, as `Visited`. But `Visited` is already a
class member, so we make things more consistent and less
parameters to pass around.

By itself, the patch has little value, but it simplifies stuff in the
#142474.

For #142461

Just for consistency, to avoid confusing conditions.

`reverse` helps to avoid tests updates as nothing is
changing for for successors count <=2.

For #142461

'goto' is essentially a shortcut for push/pop for worklist.
It can be expensive if we copy vectors, but if we move them, it
should not be an issue.

Without 'goto' it's easier to reason about the
code, when `PromoteMem2Reg::RenamePass` processes
exactly one edge at a time.

There is out of order processing of the first
successor, I keep it just to make this patch pure NFC. I'll
remove this in follow up patches.

For #142461