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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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?

No, in some cases it requires preserving values longer (and some cases end up promoting to 0). One of the test updates I fixed by inserting store of poison, it regressed by adding a whole lot of new copies 

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