[llvm] [AMDGPU] Fix PromoteAlloca size check of alloca for store (PR #72528)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Good catch! I thought in such cases the insertelement would just be eliminated (as it stores to nothing and it's a no-side-effect operation) but indeed semantics dictate it returns poison. https://llvm.org/docs/LangRef.html#insertelement-instruction

https://github.com/llvm/llvm-project/pull/72528