[llvm] AMDGPU: Use freeze poison instead of undef in alloca promotion (PR #131285)

[Carl Ritson via llvm-commits]

https://github.com/perlfu commented:

Thanks for looking at this.  I was unsure about touching `undef` when I worked on promote alloca recently.
I assume the regressions are mostly redundant instructions at the IR level which do not end up in codegen?

https://github.com/llvm/llvm-project/pull/131285