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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > Should be a generic combine. Can you generalize to N unused defs?
> 
> 
> 
> G_EXT is an opcode for AArch64's `ext` instruction. Could you please suggest how to generalize it for other targets?
> 
> 

G_EXT should be G_AARCH64_EXT then. I assumed this was shorthand for all 3 extends 

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