[llvm] [AMDGPU] Save and restore SCC using only 32-bit SGPR. (PR #68367)

[Sirish Pande via llvm-commits]

srpande wrote:

> If I understand correctly, the intent is to make the lowering of the copies from/to SCC be driven by the size of the dst/src register, instead of the wave size, to reduce register pressure when the second 32 bit reg is not needed?
> 
> If so I think the patch needs a better title/description, I initially thought it was always using a 32 bit register. Something like `Use 32 SGPR to save/restore SCC when appropriate`

That is the intent of this patch. Will change the title

https://github.com/llvm/llvm-project/pull/68367