[llvm] [GlobalISel] Remove references to rhs of shufflevector if rhs is undef (PR #115076)

[Amara Emerson via llvm-commits]

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@@ -1948,7 +1956,7 @@ def all_combines : GICombineGroup<[integer_reassoc_combines, trivial_combines,
     fsub_to_fneg, commute_constant_to_rhs, match_ands, match_ors,
     combine_concat_vector, match_addos,
     sext_trunc, zext_trunc, prefer_sign_combines, combine_shuffle_concat,
-    combine_use_vector_truncate, merge_combines]>;
+    combine_use_vector_truncate, merge_combines, combine_shuffle_undef_rhs]>;
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aemerson wrote:

Can you create a new combine group, maybe `shuffle_combines`, and put this and the `combine_shuffle_concat` combine into it? Can throw in anything else you think fits the scope there too.

https://github.com/llvm/llvm-project/pull/115076