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

[Thorsten Schütt via llvm-commits]

tschuett 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.

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