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

[Amara Emerson via llvm-commits]

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@@ -1071,18 +1071,18 @@ void applyVectorSextInReg(MachineInstr &MI, MachineRegisterInfo &MRI,
 bool matchUnmergeExtToUnmerge(MachineInstr &MI, MachineRegisterInfo &MRI,
                               Register &MatchInfo) {
   assert(MI.getOpcode() == TargetOpcode::G_UNMERGE_VALUES);
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aemerson wrote:

This assert becomes unnecessary once you have the `cast<GUnmerge>` below.

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