[X86] Remove flag setting ISD nodes from computeKnownBitsForTargetNode

[craig.topper]

Summary

The flag result is an i32 type. But its only really used for connectivity. I don't think anything even assumes a particular format. We don't ever do any real operations on it. So known bits don't help us optimize anything.

My main motivation is that the UMUL behavior is actually wrong. I was going to fix this in D36654, but then realized there was just no reason for it to be here.

[RKSimon]

LGTM

lib/Target/X86/X86ISelLowering.cpp

assert((Opc >= ISD::BUILTIN_OP_END ||
           Opc == ISD::INTRINSIC_W_CHAIN ||
           Opc == ISD::INTRINSIC_VOID) &&
          "Should use MaskedValueIsZero if you don't know whether Op"
          " is a target node!");
 
   Known.resetAll();
   switch (Opc) {
   default: break;
-  case X86ISD::ADD:
-  case X86ISD::SUB:
-  case X86ISD::ADC:
-  case X86ISD::SBB:
-  case X86ISD::SMUL:
-  case X86ISD::UMUL:
-  case X86ISD::INC:
-  case X86ISD::DEC:
-  case X86ISD::OR:
-  case X86ISD::XOR:
-  case X86ISD::AND:
-    // These nodes' second result is a boolean.
-    if (Op.getResNo() == 0)
-      break;
-    LLVM_FALLTHROUGH;
   case X86ISD::SETCC:
     Known.Zero.setBitsFrom(1);
     break;
   case X86ISD::MOVMSK: {
     unsigned NumLoBits = Op.getOperand(0).getValueType().getVectorNumElements();
     Known.Zero.setBitsFrom(NumLoBits);
     break;
   }