[InstCombine][NFC] Add tests for variable mask canonicalization in masked merge

Summary:
Masked merge has a pattern of: `((x ^ y) & M) ^ y`.
But, there is no difference between `((x ^ y) & M) ^ y` and `((x ^ y) & ~M) ^ x`,
We should canonicalize the pattern to non-inverted mask.

Differential Revision: https://reviews.llvm.org/D45663

llvm-svn: 331111