[InstCombine] Fix potentially buggy code in `((%x & C) == 0) --> %x u< (-C)` transform

[goldstein.w.n]

Summary

Its possible for C2 to have set bits that are known not-needed
(zeroable). See: D140858

+ will overflow here, | will get the right logic.

[nikic]

LGTM

[nikic]

If you don't have commit access, can you please share the "Name <email>" to use for the commit?

See also https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access for requesting commit access.

[goldstein.w.n]

In D141089#4033604, @nikic wrote:

If you don't have commit access, can you please share the "Name <email>" to use for the commit?

I don't, email: [e-mail]. Thank you :)

See also https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access for requesting commit access.

Will do, although if its all the same, wouldn't mind getting a bit more familiar
before getting write access, don't want to mess up big time :/

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

if (C2->isSignMask()) {
       Constant *Zero = Constant::getNullValue(X->getType());
       auto NewPred = isICMP_NE ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_SGE;
       return new ICmpInst(NewPred, X, Zero);
     }
 
     APInt NewC2 = *C2;
     KnownBits Know = computeKnownBits(And->getOperand(0), 0, And);
     // Set high zeros of C2 to allow matching negated power-of-2.
-    NewC2 = *C2 + APInt::getHighBitsSet(C2->getBitWidth(),
+    NewC2 = *C2 | APInt::getHighBitsSet(C2->getBitWidth(),
                                         Know.countMinLeadingZeros());
 
     // Restrict this fold only for single-use 'and' (PR10267).
     // ((%x & C) == 0) --> %x u< (-C)  iff (-C) is power of two.
     if (NewC2.isNegatedPowerOf2()) {
       Constant *NegBOC = ConstantInt::get(And->getType(), -NewC2);
       auto NewPred = isICMP_NE ? ICmpInst::ICMP_UGE : ICmpInst::ICMP_ULT;
       return new ICmpInst(NewPred, X, NegBOC);