[DAG] Avoid ComputeNumSignBits call when we know the result is unsigned

D146121 needs to set the NSW flag, but given the result is NUW then we know that the result has leading zeros, so we don't need to call ComputeNumSignBits - just reuse the existing KnownBits value instead.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -1834,8 +1834,7 @@ bool TargetLowering::SimplifyDemandedBits(
           // that the upper bits of the shift result are known to be zero,
           // which is equivalent to the narrow shift being NUW.
           if (bool IsNUW = (Known.countMinLeadingZeros() >= HalfWidth)) {
-            unsigned NumSignBits = TLO.DAG.ComputeNumSignBits(Op0, Depth + 1);
-            bool IsNSW = NumSignBits > (ShAmt + HalfWidth);
+            bool IsNSW = Known.countMinSignBits() > HalfWidth;
             SDNodeFlags Flags;
             Flags.setNoSignedWrap(IsNSW);
             Flags.setNoUnsignedWrap(IsNUW);