[llvm] [Mips] Legalize vector UNDEF instead of expanding to zero BUILD_VECTOR (PR #211503)

[via llvm-commits]

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@@ -447,6 +447,7 @@ addMSAFloatType(MVT::SimpleValueType Ty, const TargetRegisterClass *RC) {
   setOperationAction(ISD::EXTRACT_VECTOR_ELT, Ty, Legal);
   setOperationAction(ISD::INSERT_VECTOR_ELT, Ty, Legal);
   setOperationAction(ISD::BUILD_VECTOR, Ty, Custom);
+  setOperationAction(ISD::UNDEF, Ty, Legal);
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yingopq wrote:

I did not find the base class do set UNDEF to Legal for all types. 

https://github.com/llvm/llvm-project/pull/211503