Inline the fast path of plpgsql's exec_cast_value().

 Inline the fast path of plpgsql's exec_cast_value().
 
 In the common case where this function isn't actually asked to perform
 any type conversion, there's nothing it has to do beyond comparing the
 arguments.  Arrange for that part to be inlined into callers, with the
 slower path remaining out-of-line.  This seems to be good for several
 percent speedup on simple cases, with only minimal code bloat.
 
 Amit Khandekar
 
 Discussion: https://postgr.es/m/CAJ3gD9eBNrmUD7WBBLG8ohaZ485H9y+4eihQTgr+K8Lhka3vcQ@mail.gmail.com