[llvm] [AMDGPU] In instruction selector, allow copy from physical reg to s1 (PR #96157)

[Jun Wang via llvm-commits]

jwanggit86 wrote:

> isVCC needs to gain a new parameter for the partnered register in the copy. There shouldn't be any need to fixup the result after, or check register class membership

`isVCC` is used in a number of other places, e.g., inside `selectG_UADDO_USUBO_UADDE_USUBE()`. In those other contexts apparently they don't consider "the other" reg. I don't know if there even is another reg involved. So, would it be better to create a new function just for this particular case, ie., copy from phy reg to s1?

https://github.com/llvm/llvm-project/pull/96157