Additionally, make the `Loop` argument to `printLoop` const.

Loop fusion pass will use the information provided by the recent
DA patch to fuse additional legal loops, including those with
forward loop-carried dependencies.

If we have instructions in second loop's preheader which can be sunk, we
should also be adjusting PHI nodes to receive values from the fused loop's latch block.

Fixes #128600

This reverts the revert commit bf92b127d2637948f53d11a187e865aa10e2e74c.

This adds missing initialization of PeelLast in gatherPeelingPreferences.

Original message:
Generalize countToEliminateCompares to also consider peeling off the
last iteration if it eliminates a compare.

At the moment, codegen for peeling off the last iteration is quite
restrictive and callers have to make sure that the exit condition can be
adjusted when peeling and that the loop executes at least 2 iterations.

Both will be relaxed in follow-ups.

PR: https://github.com/llvm/llvm-project/pull/139551

This reverts commit bb10c3ba7f77d40a7fbfd4ac815015d3a4ae476a.

Also reverts 4f663cca15f2b53c2bc6a84d1b1f5bd81679356d:
  Revert "[LoopPeel] Make sure PeelLast is always initialized."

Revert for now to bring msan bots back to green

 https://lab.llvm.org/buildbot/#/builders/164/builds/9992
 https://lab.llvm.org/buildbot/#/builders/94/builds/7158

Generalize countToEliminateCompares to also consider peeling off the
last iteration if it eliminates a compare.

At the moment, codegen for peeling off the last iteration is quite
restrictive and callers have to make sure that the exit condition can be
adjusted when peeling and that the loop executes at least 2 iterations.

Both will be relaxed in follow-ups.

PR: https://github.com/llvm/llvm-project/pull/139551

Use `poison` instead of `undef` as a placeholder for phi entries of
unreachable predecessors.

Parameter PossiblyLoopIndependent has lost its intended purpose. This
flag is always set to true in all cases when depends() is called, hence
we want to reconsider the utility of this variable and remove it from
the function signature entirely. This is an NFC patch.

As part of the "RemoveDIs" project, BasicBlock::iterator now carries a
debug-info bit that's needed when getFirstNonPHI and similar feed into
instruction insertion positions. Call-sites where that's necessary were
updated a year ago; but to ensure some type safety however, we'd like to
have all calls to moveBefore use iterators.

This patch adds a (guaranteed dereferenceable) iterator-taking
moveBefore, and changes a bunch of call-sites where it's obviously safe
to change to use it by just calling getIterator() on an instruction
pointer. A follow-up patch will contain less-obviously-safe changes.

We'll eventually deprecate and remove the instruction-pointer
insertBefore, but not before adding concise documentation of what
considerations are needed (very few).

Identified with misc-include-cleaner.