[DAGCombiner] Masked merge: don't touch "not" xor's.

Summary:
Split off form D46031.

It seems we don't want to transform the pattern if the `xor`'s are actually `not`'s.
In vector case, this breaks `andnpd` / `vandnps` patterns.

That being said, we may want to re-visit this `not` handling, maybe in D46073.

Reviewers: spatel, craig.topper, javed.absar

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46492

llvm-svn: 331595