Refactor join_is_removable() to separate out distinctness-proving logic.

Extracted from pending unique-join patch, since this is a rather large
delta but it's simply moving code out into separately-accessible
subroutines.

I (tgl) did choose to add a bit more logic to rel_supports_distinctness,
so that it verifies that there's at least one potentially usable unique
index rather than just checking indexlist != NIL.  Otherwise there's
no functional change here.

David Rowley