[DAG] computeKnownBits - Replace ISD::MUL handling with the common KnownBits::computeForMul implementation

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2879,20 +2879,7 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
   case ISD::MUL: {
     Known = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
     Known2 = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
-
-    // If low bits are zero in either operand, output low known-0 bits.
-    // Also compute a conservative estimate for high known-0 bits.
-    // More trickiness is possible, but this is sufficient for the
-    // interesting case of alignment computation.
-    unsigned TrailZ = Known.countMinTrailingZeros() +
-                      Known2.countMinTrailingZeros();
-    unsigned LeadZ =  std::max(Known.countMinLeadingZeros() +
-                               Known2.countMinLeadingZeros(),
-                               BitWidth) - BitWidth;
-
-    Known.resetAll();
-    Known.Zero.setLowBits(std::min(TrailZ, BitWidth));
-    Known.Zero.setHighBits(std::min(LeadZ, BitWidth));
+    Known = KnownBits::computeForMul(Known, Known2);
     break;
   }
   case ISD::UDIV: {