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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> `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?

You could either add another variant for copies, or add an optional parameter for the paired register. The other uses do not need to consider physical registers 

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