[llvm] [GlobalISel] Remove dead VRegs after instruction selection (PR #73892)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm what do you think? After these experiments and the discussion can we return to the @qcolombet's suggestion to traverse only alive registers as a fix: https://reviews.llvm.org/D157458?id=548421#4579446? I think it is better than skipping registers without reg class.

OK, I see the issue is this X86 pass is oddly iterating all registers by number and not processing by instruction. I think it makes sense to just skip dead registers there 

https://github.com/llvm/llvm-project/pull/73892