[SelectionDAG] Optional handling of UNDEF elements in matchBinaryPredicate (part 2 of 2)

Now that SimplifyDemandedBits/SimplifyDemandedVectorElts is simplifying vector elements, we're seeing more constant BUILD_VECTOR containing undefs.

This patch provides opt-in support for UNDEF elements in matchBinaryPredicate, passing NULL instead of the result ConstantSDNode* argument.

I've updated the (or (and X, c1), c2) -> (and (or X, c2), c1|c2) fold to demonstrate its use, which I believe is safe for undef cases.

Differential Revision: https://reviews.llvm.org/D55822

llvm-svn: 349629