[llvm] [AMDGPU] Implement moveToVALU for S_CSELECT_B64 (PR #70352)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Just wondering if there is still something left to implement

Yes! The instruction selection parts of [D109159](https://reviews.llvm.org/D109159) still need to be unreverted. But I don't know exactly what problem caused them to be reverted in the first place, so I would like to proceed in small steps.

https://github.com/llvm/llvm-project/pull/70352