[InstCombine] Only requite not-undef in select equiv fold

As the comment already indicates, only replacement with undef
is problematic, as it introduces an additional use of undef.
Use the correct ValueTracking helper.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1294,8 +1294,7 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
   // X == Y ? X : Z with X == Y ? Y : Z, as that would lead to an infinite
   // replacement cycle.
   Value *CmpLHS = Cmp.getOperand(0), *CmpRHS = Cmp.getOperand(1);
-  if (TrueVal != CmpLHS &&
-      isGuaranteedNotToBeUndefOrPoison(CmpRHS, SQ.AC, &Sel, &DT)) {
+  if (TrueVal != CmpLHS && isGuaranteedNotToBeUndef(CmpRHS, SQ.AC, &Sel, &DT)) {
     if (Value *V = simplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, SQ,
                                           /* AllowRefinement */ true))
       // Require either the replacement or the simplification result to be a
@@ -1316,8 +1315,7 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
       if (replaceInInstruction(TrueVal, CmpLHS, CmpRHS))
         return &Sel;
   }
-  if (TrueVal != CmpRHS &&
-      isGuaranteedNotToBeUndefOrPoison(CmpLHS, SQ.AC, &Sel, &DT))
+  if (TrueVal != CmpRHS && isGuaranteedNotToBeUndef(CmpLHS, SQ.AC, &Sel, &DT))
     if (Value *V = simplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, SQ,
                                           /* AllowRefinement */ true))
       if (isa<Constant>(CmpLHS) || isa<Constant>(V))

llvm/test/Transforms/InstCombine/select.ll

@@ -2852,8 +2852,7 @@ define i8 @select_replacement_sub_noundef_but_may_be_poison(i8 %x, i8 noundef %y
 ; CHECK-LABEL: @select_replacement_sub_noundef_but_may_be_poison(
 ; CHECK-NEXT:    [[Y:%.*]] = shl nuw i8 [[YY:%.*]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[Y]], [[X:%.*]]
-; CHECK-NEXT:    [[SUB:%.*]] = sub i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 [[SUB]], i8 [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP]], i8 0, i8 [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %y = shl nuw i8 %yy, 1