[TargetLowering] add special-case for demanded bits analysis of 'not'

We treat bitwise 'not' as a special operation and try not to reduce its all-ones mask. 
Presumably, this is because a 'not' may be cheaper than a generic 'xor' or it may get
folded into another logic op if the target has those. However, if we can remove a logic
instruction by changing the xor's constant mask value, that should always be a win.

Note that the IR version of SimplifyDemandedBits() does not treat 'not' as a special-case
currently (although that's marked with a FIXME). So if you run this IR through -instcombine,
you should get the same end result. I'm hoping to add a different backend transform that 
will expose this problem though, so I need to solve this first.

Differential Revision: https://reviews.llvm.org/D27356

llvm-svn: 288676