[llvm] [AMDGPU] Combine prng(undef) -> undef (PR #154160)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> I'm not sure this is safe, but we should only bother doing this in instcombine if it is. This isn't a pattern likely to appear in lowering

It is basically 'garbage in - garbage out'. You are reading garbage and produce garbage as a random number.

https://github.com/llvm/llvm-project/pull/154160