[llvm] [AMDGPU] Fix nondeterminism in SIFixSGPRCopies (PR #70644)

[Marek Olšák via llvm-commits]

marekolsak wrote:

I'm the original reporter of the bug in AMD's internal issue tracker.

I verify that this resolves the issue, which caused compiled shaders to be randomly influenced by shaders compiled before them.

https://github.com/llvm/llvm-project/pull/70644