[llvm] [GlobalIsel] Improve poison analysis (PR #93731)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Should we add COPY to `canCreateUndefOrPoison` with false?

Yes, COPY should never be able to introduce poison 

> Then MachineInstr *RegDef = MRI.getVRegDef(Reg);might fail.

Don't follow. The only case it should be able to fail is for undef use operands (which I think should be a verifier error. We should not permit undef uses for IsSSA functions)

https://github.com/llvm/llvm-project/pull/93731