Identified with misc-include-cleaner.

[FixIrreducible] Use CycleInfo instead of a custom SCC traversal

1. CycleInfo efficiently locates all cycles in a single pass, while the
SCC is
   repeated inside every natural loop.

2. CycleInfo provides a hierarchy of irreducible cycles, and the new
implementation transforms each cycle in this hierarchy separately
instead of
reducing an entire irreducible SCC in a single step. This reduces the
number
of control-flow paths that pass through the header of each newly created
loop. This is evidenced by the reduced number of predecessors on the
"guard"
blocks in the lit tests, and fewer operands on the corresponding PHI
nodes.

3. When an entry of an irreducible cycle is the header of a child
natural loop,
the original implementation destroyed that loop. This is now preserved,
   since the incoming edges on non-header entries are not touched.

4. In the new implementation, if an irreducible cycle is a superset of a
natural
loop with the same header, then that natural loop is destroyed and
replaced
   by the newly created loop.

CreateControlFlowHub is a method that redirects control flow edges from a set of
incoming blocks to a set of outgoing blocks through a new set of "guard" blocks.
This is now refactored into a separate file with one enhancement: The input to
the method is now a set of branches rather than two sets of blocks.

The original implementation reroutes every edge from incoming blocks to outgoing
blocks. But it is possible that for some incoming block InBB, some successor S
might be in the set of outgoing blocks, but that particular edge should not be
rerouted. The new implementation makes this possible by allowing the user to
specify the targets of each branch that need to be rerouted.

This is needed when improving the implementation of FixIrreducible #101386.
Current use in FixIrreducible does not demonstrate this finer control over the
edges being rerouted. But in UnifyLoopExits, when only one successor of an
exiting block is an exit block, this refinement now reroutes only the relevant
control-flow through the edge; the non-exit successor is not rerouted. This
results in fewer branches and PHI nodes in the hub.

Some passes has limitation that only support simple terminators:
branch/unreachable/return. Right now, they ask the pass manager to add
LowerSwitch pass to eliminate `switch`. Let's manage such kind of pass
dependency by ourselves. Also add the assertion in the related passes.

Identified with readability-qualified-auto.

Identified with readability-qualified-auto.

The modified function was incorrectly (not unnecessarily) ignoring grandchild
loops, and this change fixes the bug. In particular, this fixes the handling of
the loop { inner, body }. The TODO in the same function is talking about the b1
self loop, which may be "unnecessarily" lost, but that is a different issue.

Estimation on the impact on preprocessor output:
before: 1065307662
after:  1064800684

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D120741