[llvm-branch-commits] [LPM][LegacyPM] Reenable LCSSA Verification (PR #191667)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/191667

This was disabled about a decade ago due to issues with LoopSink.
LoopSink has since had its LegacyPM version removed and is now a
function pass due to not needing too much loop infrastructure. So we can
try enabling this again to prevent backsliding on important cases while
we work on switching to the NewPM which does enforce these things.

Eventually we will want to add assertions here for LoopStrengthReduce,
but given it does not correctly preserve LCSSA, postpone that for now.