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

[Evgenii Kudriashov via llvm-commits]

e-kud wrote:

> I agree that removing such footgun is good, but it shouldn't come with a high price. Most places traverse the MachineOperand so they are not exposed to this problem.

@qcolombet yes, I agree, moreover, there is only one place among all targets with GlobalISel where we hit the problem. I think we potentially create a precedent of workarounding GlobalISel selection results in backends. It doesn't sound like a good practice. However there is a fine line between workaround and correct traversal.

@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.

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