[SelectionDAG] Use KnownBits::countMinSignBits() to simplify the end of ComputeNumSignBits.

[craig.topper]

Summary

This matches what is done in ValueTracking.cpp

[RKSimon]

LGTM - cheers

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

unsigned NumBits =
         TLI->ComputeNumSignBitsForTargetNode(Op, DemandedElts, *this, Depth);
     if (NumBits > 1)
       FirstAnswer = std::max(FirstAnswer, NumBits);
   }
 
   // Finally, if we can prove that the top bits of the result are 0's or 1's,
   // use this information.
   KnownBits Known = computeKnownBits(Op, DemandedElts, Depth);
-
-  APInt Mask;
-  if (Known.isNonNegative()) {        // sign bit is 0
-    Mask = Known.Zero;
-  } else if (Known.isNegative()) {  // sign bit is 1;
-    Mask = Known.One;
-  } else {
-    // Nothing known.
-    return FirstAnswer;
-  }
-
-  // Okay, we know that the sign bit in Mask is set.  Use CLO to determine
-  // the number of identical bits in the top of the input value.
-  Mask <<= Mask.getBitWidth()-VTBits;
-  return std::max(FirstAnswer, Mask.countLeadingOnes());
+  return std::max(FirstAnswer, Known.countMinSignBits());
 }
 
 unsigned SelectionDAG::ComputeMinSignedBits(SDValue Op, unsigned Depth) const {
   unsigned SignBits = ComputeNumSignBits(Op, Depth);
   return Op.getScalarValueSizeInBits() - SignBits + 1;
 }
 
 unsigned SelectionDAG::ComputeMinSignedBits(SDValue Op,