[DAG] add fold for masked negated extended bool

The non-obvious motivation for adding this fold (which already happens in InstCombine)
is that we want to canonicalize IR towards select instructions and canonicalize DAG 
nodes towards boolean math. So we need to recreate some folds in the DAG to handle that
change in direction. 

An interesting implementation difference for cases like this is that InstCombine
generally works top-down while the DAG goes bottom-up. That means we need to detect 
different patterns. In this case, the SimplifyDemandedBits fold prevents us from 
performing a zext to sext fold that would then be recognized as a negation of a sext. 

llvm-svn: 283900