Heavily refactor btsortsupport_worker.

Prior to commit 4ea51cdf, this function
only had one job, which was to decide whether we could avoid trampolining
through the fmgr layer when performing sort comparisons.  As of that
commit, it has a second job, which is to decide whether we can use
abbreviated keys.  Unfortunately, those two tasks are somewhat intertwined
in the existing coding, which is likely why neither Peter Geoghegan nor
I noticed prior to commit that this calls pg_newlocale_from_collation() in
cases where it didn't previously.  The buildfarm noticed, though.

To fix, rewrite the logic so that the decision as to which comparator to
use is more cleanly separated from the decision about abbreviation.