[SelectionDAG] Teach GetDemandedBits to look at the known zeros of the LHS when handl...

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354514 91177308-0d34-0410-b5e6-96231b3b80d8