[clang] [llvm] [opt][AMDGPU] Add pass to handle AMDGCN pseudo-intrinsics target specific info), start with `llvm.amdgcn.wavefrontsize` (PR #114481)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I think the current use of this intrinsic in 'generic' IR is sound so long as it's not guarding anything ABI related. Right now it's just used for loop bounds and array offsets pretty much. Though long-term I agree that it's probably most sound to just put  these as separate builds, but that would require all of our device runtime stuff going through my interface / using multilibs.

I think the only thing important here is that we don't do this prematurely, i.e. `clang --target=amdgcn-amd-amdhsa -c -flto` does not fold it since it's before codegen.

@JonChesterfield was working on 'generic' IR as well, so maybe he has some opinions.

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