[SelectionDAG] define binops as a superset of commutative binops

The test diffs show improved vector narrowing for integer min/max opcodes because
those were all absent from the list. I'm not sure if we can expose functional diffs
for all of the moved/added opcodes though.

It seems like we are missing an AVX512 opportunity to use 256-bit ops in place of
512-bit ops on some tests/targets, but I think that can be a follow-up.

Preliminary steps to make sure the callers are not misusing these queries:
rL361268
rL361547

Differential Revision: https://reviews.llvm.org/D62191

llvm-svn: 361701

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