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

[Carl Ritson via llvm-commits]

https://github.com/perlfu approved this pull request.

I think this should be fine.

I did a static compile test on common graphics shaders and saw almost no changes.  Changes I did see were not really note worthy.

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