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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I thought the problematic context was when registers are only referenced by debug instructions? Can we do something about those specifically instead?

More concretely, dead registers are not a problem. They are only a problem if referenced by an instruction. All instructions should be processing all operands. If you have a register with an unset class, it was mis-selected. We shouldn't need to inspect the full set of registers, all used registers should be naturally encountered during the select process 

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