[GlobalISel] GISelKnownBits::computeKnownBitsImpl - Replace TargetOpc…

…ode::G_MUL handling with the common KnownBits::computeForMul implementation

Avoid code duplication

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

@@ -287,20 +287,7 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
                          Depth + 1);
     computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, 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 TargetOpcode::G_SELECT: {