[InstCombine][NFC] masked-merge: commutativity tests: ensure the ordering.

This was intended since initially, but i did not really think
about it, and did not know how to force that. Now that the
xor->or fold is working (patch upcoming), this came up
to improve the test coverage.

A followup for rL330003, rL330007
https://bugs.llvm.org/show_bug.cgi?id=6773

llvm-svn: 330039