[InstCombine] remove unnecessary one-use check from add-xor transform

Pre-conditions seem to be optimal, but we don't need a use check
because we are only replacing an add with a sub.

https://rise4fun.com/Alive/hzN

  Pre: (~C1 | C2 == -1) && isPowerOf2(C2+1)
  %m = and i8 %x, C1
  %f = xor i8 %m, C2
  %r = add i8 %f, C3
  =>
  %r = sub i8 C2 + C3, %m

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1296,9 +1296,9 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
         return BinaryOperator::CreateAShr(NewShl, ShAmt);
       }
 
-      // If this is a xor that was canonicalized from a sub, turn it back into
-      // a sub and fuse this add with it.
-      if (LHS->hasOneUse() && (XorRHS->getValue()+1).isPowerOf2()) {
+      // If X has no high-bits above the xor mask set:
+      // add (xor X, LowMaskC), C --> sub (LowMaskC + C), X
+      if ((XorRHS->getValue() + 1).isPowerOf2()) {
         KnownBits LHSKnown = computeKnownBits(XorLHS, 0, &I);
         if ((XorRHS->getValue() | LHSKnown.Zero).isAllOnesValue())
           return BinaryOperator::CreateSub(ConstantExpr::getAdd(XorRHS, CI),

llvm/test/Transforms/InstCombine/sub-xor.ll

@@ -55,7 +55,7 @@ define i32 @xor_add_extra_use(i32 %x) {
 ; CHECK-NEXT:    [[AND:%.*]] = and i32 [[X:%.*]], 31
 ; CHECK-NEXT:    [[XOR:%.*]] = xor i32 [[AND]], 31
 ; CHECK-NEXT:    call void @use(i32 [[XOR]])
-; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw i32 [[XOR]], 42
+; CHECK-NEXT:    [[ADD:%.*]] = sub nuw nsw i32 73, [[AND]]
 ; CHECK-NEXT:    ret i32 [[ADD]]
 ;
   %and = and i32 %x, 31