[llvm] [AMDGPU] Fold compares of S_CSELECT of two different constants (PR #227772)

[via llvm-commits]

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LU-JOHN wrote:

foldableSelect should be inlined.
The remaining use of foldableSelect only exploits:     
`//    sX = s_cselect_b64 (non-zero imm), 0`
It can be re-written to exploit:
`//    sX = s_cselect_b64 A, B`
where A!=B and one of A or B is 0.  It can reuse the `getSelectConstants` from this PR.

https://github.com/llvm/llvm-project/pull/227772