[InstCombine] Improve coverage of foldSelectValueEquivalence for non-constants

If f(Y) simplifies to Y, replace with Y. This requires Y to be
non-undef.

Closes #94719

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1310,7 +1310,7 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
 
       // If NewOp is a constant and OldOp is not replace iff NewOp doesn't
       // contain and undef elements.
-      if (match(NewOp, m_ImmConstant())) {
+      if (match(NewOp, m_ImmConstant()) || NewOp == V) {
         if (isGuaranteedNotToBeUndef(NewOp, SQ.AC, &Sel, &DT))
           return replaceOperand(Sel, Swapped ? 2 : 1, V);
         return nullptr;

llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll

@@ -6,8 +6,7 @@ declare void @use.i8(i8)
 define i8 @replace_with_y_noundef(i8 %x, i8 noundef %y, i8 %z) {
 ; CHECK-LABEL: @replace_with_y_noundef(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[AND]], i8 [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[Y]], i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %cmp = icmp eq i8 %x, %y
@@ -20,8 +19,7 @@ define i8 @replace_with_x_noundef(i8 noundef %x, i8 %y, i8 %z) {
 ; CHECK-LABEL: @replace_with_x_noundef(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    call void @use.i1(i1 [[CMP]])
-; CHECK-NEXT:    [[AND:%.*]] = or i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[Z:%.*]], i8 [[AND]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[Z:%.*]], i8 [[X]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %cmp = icmp ne i8 %x, %y