[InstCombine] Add test for #89669 (NFC)

Author: nikic

llvm/test/Transforms/InstCombine/sub-of-negatible.ll

@@ -1385,6 +1385,19 @@ define i8 @dont_negate_ordinary_select(i8 %x, i8 %y, i8 %z, i1 %c) {
   ret i8 %t1
 }
 
+; FIXME: This is a miscompile.
+define <2 x i32> @negate_select_of_negation_poison(<2 x i1> %c, <2 x i32> %x) {
+; CHECK-LABEL: @negate_select_of_negation_poison(
+; CHECK-NEXT:    [[NEG:%.*]] = sub <2 x i32> <i32 0, i32 poison>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = select <2 x i1> [[C:%.*]], <2 x i32> [[X]], <2 x i32> [[NEG]]
+; CHECK-NEXT:    ret <2 x i32> [[TMP1]]
+;
+  %neg = sub <2 x i32> <i32 0, i32 poison>, %x
+  %sel = select <2 x i1> %c, <2 x i32> %neg, <2 x i32> %x
+  %neg2 = sub <2 x i32> zeroinitializer, %sel
+  ret <2 x i32> %neg2
+}
+
 ; Freeze is transparent as far as negation is concerned
 define i4 @negate_freeze(i4 %x, i4 %y, i4 %z) {
 ; CHECK-LABEL: @negate_freeze(