[TargetLowering] SimplifyDemandedBits ADD/SUB/MUL - correctly inherit...

[TargetLowering] SimplifyDemandedBits ADD/SUB/MUL - correctly inherit SDNodeFlags from the original node.

Just disable NSW/NUW flags. This matches what we're already doing for the other situations for these nodes, it was just missed for the demanded constant case.

Noticed by inspection - confirmed in offline discussion with @spatel. I've checked we have test coverage in the x86 extract-bits.ll and extract-lowbits.ll tests

llvm-svn: 370497