[llvm] [AArch64][GlobalISel] Combine G_EXTRACT_VECTOR_ELT and G_BUILD_VECTOR sequences into G_SHUFFLE_VECTOR (PR #110545)

[Matt Arsenault via llvm-commits]

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@@ -4205,6 +4218,76 @@ void CombinerHelper::applyExtractVecEltBuildVec(MachineInstr &MI,
   replaceSingleDefInstWithReg(MI, Reg);
 }
 
+bool CombinerHelper::matchCombineExtractToShuffle(
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arsenm wrote:

I suggest to follow along with the DAG version; you'll want something similar to buildLegalVectorShuffle, which will avoid creating shuffles which will just be broken up again https://github.com/llvm/llvm-project/blob/33c14f19656c751bbbc083e4a168ab898e583bfd/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L23501

https://github.com/llvm/llvm-project/pull/110545