[InstCombine] add test for miscompile from select value equivalence; NFC

The new test is reduced from:
https://llvm.org/PR49832
...but we already show a potential miscompile in the existing test too.

Author: rotateright

llvm/test/Transforms/InstCombine/select-binop-cmp.ll

@@ -551,6 +551,8 @@ define i32 @select_xor_icmp_bad_6(i32 %x, i32 %y, i32 %z) {
   ret i32 %C
 }
 
+; FIXME: Value equivalence substitution is all-or-nothing, so needs a scalar compare.
+
 define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
 ; CHECK-LABEL: @select_xor_icmp_vec_bad(
 ; CHECK-NEXT:    [[A:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 5, i8 3>
@@ -564,6 +566,18 @@ define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z)
   ret <2 x i8>  %C
 }
 
+; FIXME: Value equivalence substitution is all-or-nothing, so needs a scalar compare.
+
+define <2 x i32> @vec_select_no_equivalence(<2 x i32> %x) {
+; CHECK-LABEL: @vec_select_no_equivalence(
+; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+;
+  %x10 = shufflevector <2 x i32> %x, <2 x i32> undef, <2 x i32> <i32 1, i32 0>
+  %cond = icmp eq <2 x i32> %x, zeroinitializer
+  %s = select <2 x i1> %cond, <2 x i32> %x10, <2 x i32> %x
+  ret <2 x i32> %s
+}
+
 ; Folding this would only be legal if we sanitized undef to 0.
 define <2 x i8> @select_xor_icmp_vec_undef(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
 ; CHECK-LABEL: @select_xor_icmp_vec_undef(