[InstSimplify] Add a test for folding comparison with a undef vector (NFC)

This is to fix https://reviews.llvm.org/D93990#2666922

Author: aqjune

llvm/test/Transforms/InstSimplify/icmp-constant.ll

@@ -1065,3 +1065,13 @@ bb3:
   %m = mul nuw i8 %x, 0
   br label %bb2
 }
+
+
+define <2 x i1> @heterogeneous_constvector(<2 x i8> %x) {
+; CHECK-LABEL: @heterogeneous_constvector(
+; CHECK-NEXT:    [[C:%.*]] = icmp ult <2 x i8> [[X:%.*]], <i8 undef, i8 poison>
+; CHECK-NEXT:    ret <2 x i1> [[C]]
+;
+  %c = icmp ult <2 x i8> %x, <i8 undef, i8 poison>
+  ret <2 x i1> %c
+}