[InstCombine] fix potential miscompile in select value equivalence

As shown in the example based on:
https://llvm.org/PR49832
...and the existing test, we can't substitute
a vector value because the equality compare
replacement that we are attempting requires
that the comparison is true for the entire
value. Vector select can be partly true/false.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1124,7 +1124,10 @@ static Instruction *canonicalizeAbsNabs(SelectInst &Sel, ICmpInst &Cmp,
 /// TODO: Wrapping flags could be preserved in some cases with better analysis.
 Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
                                                           ICmpInst &Cmp) {
-  if (!Cmp.isEquality())
+  // Value equivalence substitution requires an all-or-nothing replacement.
+  // It does not make sense for a vector compare where each lane is chosen
+  // independently.
+  if (!Cmp.isEquality() || Cmp.getType()->isVectorTy())
     return nullptr;
 
   // Canonicalize the pattern to ICMP_EQ by swapping the select operands.

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

@@ -551,12 +551,12 @@ 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.
+; 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>
-; CHECK-NEXT:    [[B:%.*]] = xor <2 x i8> [[Z:%.*]], <i8 5, i8 3>
+; CHECK-NEXT:    [[B:%.*]] = xor <2 x i8> [[X]], [[Z:%.*]]
 ; CHECK-NEXT:    [[C:%.*]] = select <2 x i1> [[A]], <2 x i8> [[B]], <2 x i8> [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x i8> [[C]]
 ;
@@ -566,11 +566,14 @@ 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.
+; 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:%.*]]
+; CHECK-NEXT:    [[X10:%.*]] = shufflevector <2 x i32> [[X:%.*]], <2 x i32> undef, <2 x i32> <i32 1, i32 0>
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq <2 x i32> [[X]], zeroinitializer
+; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[COND]], <2 x i32> [[X10]], <2 x i32> [[X]]
+; CHECK-NEXT:    ret <2 x i32> [[S]]
 ;
   %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