[llvm] Reapply [AMDGPU] SIFixSgprCopies should not process twice VGPR to SGPR copies inserted by PHI preprocessing. (PR #135243)

[Konstantin Zhuravlyov via llvm-commits]

https://github.com/kzhuravl approved this pull request.

Re-apply looks good, assuming all the issues that caused the revert are fixed, I just glanced through a couple of tests.

https://github.com/llvm/llvm-project/pull/135243