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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> If we `COPY` from a physical register, we might invoke `canCreateUndefOrPoison` with a physical register. Then we invoke `MRI.getVRegDef(Reg)` where `Reg` is a physical register. It will fail.

Yes, you just avoid doing that. It's just return true if copy and bypass the getVRegDef 


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