[llvm] [GlobalIsel] Combine G_EXTRACT_VECTOR_ELT (PR #85321)

[Amara Emerson via llvm-commits]

https://github.com/aemerson commented:

We should avoid unnecessarily calling `getOpcodeDef()` on a register. Instead do a lookup of the def once with `getDefIgnoringCopies()` and then use `dyn_cast<G

https://github.com/llvm/llvm-project/pull/85321