[llvm] [AArch64][GlobalISel] Add combine for build_vector(unmerge, unmerge, undef, undef) (PR #165539)

[Matt Arsenault via llvm-commits]

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@@ -332,6 +332,13 @@ def combine_mul_cmlt : GICombineRule<
   (apply [{ applyCombineMulCMLT(*${root}, MRI, B, ${matchinfo}); }])
 >;
 
+def combine_build_unmerge : GICombineRule<
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arsenm wrote:

This is not target specific 

https://github.com/llvm/llvm-project/pull/165539