Move clause_sides_match_join() into restrictinfo.h

Move clause_sides_match_join() into restrictinfo.h

Two near-identical copies of clause_sides_match_join() existed in
joinpath.c and analyzejoins.c.  Deduplicate this by moving the function
into restrictinfo.h.

It isn't quite clear that keeping the inline property of this function
is worthwhile, but this commit is just an exercise in code
deduplication.  More effort would be required to determine if the inline
property is worth keeping.

Author: James Hunter <james.hunter.pg@gmail.com>
Discussion: https://postgr.es/m/CAJVSvF7Nm_9kgMLOch4c-5fbh3MYg%3D9BdnDx3Dv7Fcb64zr64Q%40mail.gmail.com