[DAGCombiner] Prevent unfoldMaskedMerge from creating an AND with two inverted inputs.

It's possible that the mask is already a NOT. At least if InstCombine
hasn't canonicalized the input. In that case we will form an ANDN with
X instead of with Y. So we don't need to worry about Y being a constant.

We might need to check that X isn't a constant instead, but we don't
have a test case for that yet.

This fixes a size regression found when trying to enable this combine
for RISCV in D113937.

Differential Revision: https://reviews.llvm.org/D113948