[llvm] [AArch64][GlobalISel] Lower G_BUILD_VECTOR to G_INSERT_VECTOR_ELT (PR #105686)

[Matt Arsenault via llvm-commits]

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@@ -290,6 +290,13 @@ def combine_mul_cmlt : GICombineRule<
   (apply [{ applyCombineMulCMLT(*${root}, MRI, B, ${matchinfo}); }])
 >;
 
+def lower_build_insert_vec_elt : GICombineRule<
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arsenm wrote:

Bump on this, why is this an optimization combine and not in LegalizerHelper? The title says its lowering, but this is not 

https://github.com/llvm/llvm-project/pull/105686