[llvm-branch-commits] [LSR] Preserve LCSSA in SCEVRewriter (PR #191665)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/191665

This is necessery to fix some regressions when switching to the NewPM
and seems to improve optimization quality in some cases due to LSR
currently not understanding loop nests. This patch just enables LCSSA
preservation for SCEVRewriter and updates all the relevant tests. There
are some further fixes that are needed to get this fully working that
will be included in follow up patches.

Similar to #185373 (although without follow up fixes and a regression
test).

Regression test added for the specific NewPM case noticed is in
Transforms/LoopStrengthReduce/X86/lcssa-preservation-regression.ll
(does not reproduce without the target triple).