[llvm] [GlobalISel][AArch64] Combine unmerge(G_EXT v, undef) to unmerge(v). (PR #65263)

[Amara Emerson via llvm-commits]

https://github.com/aemerson approved this pull request.

LGTM with minor changes. We can actually rename the G_EXT and friends to specify AArch64 as well in another change.

https://github.com/llvm/llvm-project/pull/65263