[llvm] [AMDGPU][CodeGen] Post-RA VGPR MSB group optimization pass (PR #222666)

[Matt Arsenault via llvm-commits]

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arsenm wrote:

I'd leave the opt level check out, we really don't do much with -O3. Also put this in addPreRewrite?

https://github.com/llvm/llvm-project/pull/222666