[clang] [llvm] [llvm][AMDGPU] Fold `llvm.amdgcn.wavefrontsize` early (PR #114481)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

LGTM once CI passes. Thanks Matt for making this less awful.

https://github.com/llvm/llvm-project/pull/114481