[SelectionDAG] Remove SUBC handling from computeKnownBits

[craig.topper]

Summary

The code that computes the known bits wasn't taking into account the possibility of an incoming carry. So I don't think this code was valid. Certainly not the code that assumed the number of low zero bits in each input determined the low zero bits of the output.

I found this purely by inspection so I don't have a test case.

[efriedma]

SUBC doesn't have an incoming carry...

[craig.topper]

Oops. SUBE would be the one with carry in. Nevermind

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

if (Op.getResNo() == 1) {
       // If we know the result of a setcc has the top bits zero, use this info.
       if (TLI->getBooleanContents(Op.getOperand(0).getValueType()) ==
               TargetLowering::ZeroOrOneBooleanContent &&
           BitWidth > 1)
         Known.Zero.setBitsFrom(1);
       break;
     }
     LLVM_FALLTHROUGH;
-  case ISD::SUB:
-  case ISD::SUBC: {
+  case ISD::SUB: {
     if (ConstantSDNode *CLHS = isConstOrConstSplat(Op.getOperand(0))) {
       // We know that the top bits of C-X are clear if X contains less bits
       // than C (i.e. no wrap-around can happen).  For example, 20-X is
       // positive if we can prove that X is >= 0 and < 16.
       if (CLHS->getAPIntValue().isNonNegative()) {
         unsigned NLZ = (CLHS->getAPIntValue()+1).countLeadingZeros();
         // NLZ can't be BitWidth with no sign bit
         APInt MaskV = APInt::getHighBitsSet(BitWidth, NLZ+1);