[llvm] [AMDGPU] Use 32-bit SGPR to save/restore of SCC (PR #68367)

[Sirish Pande via llvm-commits]

srpande wrote:

> SIFixSGPRCopies is supposed to fix "illegal" VGPR->SGPR copies. Apparently it also fixes VGPR<->SCC copies. But I do not think it should touch SGPR<->SCC copies. Those are perfectly legal and will be handled by SIInstrInfo::copyPhysReg.
> 
> So a better fix would be to teach fixSCCCopies not to touch SGPR<->SCC copies.

I can open another JIRA ticket to fix SIFixSGPRCopies. Not only does SIFixSGPRCopies needs to be rewritten for SCC Copies, there is also` AMDGPUInstructionSelector::selectCOPY` that does the same thing that copyPhysRegs does. We can address all that in the new JIRA.

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