Improve planning of btree index scans using ScalarArrayOpExpr quals.

 Improve planning of btree index scans using ScalarArrayOpExpr quals.
 
 Since we taught btree to handle ScalarArrayOpExpr quals natively (commit
 9e8da0f75731aaa7605cf4656c21ea09e84d2eb1), the planner has always included
 ScalarArrayOpExpr quals in index conditions if possible.  However, if the
 qual is for a non-first index column, this could result in an inferior plan
 because we can no longer take advantage of index ordering (cf. commit
 807a40c551dd30c8dd5a0b3bd82f5bbb1e7fd285).  It can be better to omit the
 ScalarArrayOpExpr qual from the index condition and let it be done as a
 filter, so that the output doesn't need to get sorted.  Indeed, this is
 true for the query introduced as a test case by the latter commit.
 
 To fix, restructure get_index_paths and build_index_paths so that we
 consider paths both with and without ScalarArrayOpExpr quals in non-first
 index columns.  Redesign the API of build_index_paths so that it reports
 what it found, saving useless second or third calls.
 
 Report and patch by Andrew Gierth (though rather heavily modified by me).
 Back-patch to 9.2 where this code was introduced, since the issue can
 result in significant performance regressions compared to plans produced
 by 9.1 and earlier.