[SelectionDAG] Teach GetDemandedBits to look at the known zeros of the LHS when handling ISD::AND

If the LHS has known zeros, then the RHS immediate mask might have been simplified to remove those bits.

This patch adds a call to computeKnownBits to get the known zeroes to handle that possibility. I left an early out to skip the call if all of the demanded bits are set in the mask.

Differential Revision: https://reviews.llvm.org/D58464

llvm-svn: 354514