Really check that the bits that will become zero are actually already zero

before eliminating the operation that zeros them. This fixes rdar://8739316.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121353 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -168,9 +168,8 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
     // We can always turn lshr(c1)+shl(c2) -> lshr(c3)+and(c4), but it isn't
     // profitable unless we know the and'd out bits are already zero.
     if (CI->getZExtValue() > NumBits) {
-      unsigned LowBits = CI->getZExtValue() - NumBits;
       if (MaskedValueIsZero(I->getOperand(0),
-                          APInt::getLowBitsSet(TypeWidth, LowBits) << NumBits))
+                            APInt::getLowBitsSet(TypeWidth, NumBits)))
         return true;
     }