[llvm] [LSR] All known LCSSA fixes (PR #193824)

[Vikram Hegde via llvm-commits]

vikramRH wrote:

@boomanaiden154 does it make sense to have a fix right now that at least keeps behavior similar to legacy (either make LSR a function pass now until its ready to preserve LCSSA without regressions or make LCSSA optional as @nikic earlier suggsested) ? especially considering  we see quite a few regressions right now and this being a major blocker towards enabling NPM ? (this right now stands to be the only blocker for AMDGPU backend at least)

https://github.com/llvm/llvm-project/pull/193824