Fix a comparison function to actually be a SWO so that it conforms to

the spec required by std::sort and friends.

Ordering things this way also dramatically simplifies the code as
short-circuit ensures we can skip all of the negative tests.

I've left one FIXME where we're establishing a fairly arbitrary
ordering. Previously, the function compared all types as equal except
for the ones it explicitly handled, but it didn't delegate correctly to
the atomflags when doing so, and so it would fail to be a SWO. The two
possible fixes are to stop comparing the atom flags entirely, or to
establish some arbitrary ordering of the types.

Since it was pure luck which ordering of unequal types we ended up with
previously (the caller was std::sort, not std::stable_sort) I chose to
make the ordering explicit and guaranteed. This seems like the best
conservative approach as I suspect we would want to switch to
stable_sort otherwise in order to have deterministic output.

Differential Revision: http://reviews.llvm.org/D8266

llvm-svn: 231968