Fix corner-case failure in match_pattern_prefix().

The planner's optimization code for LIKE and regex operators could
error out with a complaint like "no = operator for opfamily NNN"
if someone created a binary-compatible index (for example, a
bpchar_ops index on a text column) on the LIKE's left argument.

This is a consequence of careless refactoring in commit 74dfe58a5.
The old code in match_special_index_operator only accepted specific
combinations of the pattern operator and the index opclass, thereby
indirectly guaranteeing that the opclass would have a comparison
operator with the same LHS input type as the pattern operator.
While moving the logic out to a planner support function, I simplified
that test in a way that no longer guarantees that.  Really though we'd
like an altogether weaker dependency on the opclass, so rather than
put back exactly the old code, just allow lookup failure.  I have in
mind now to rewrite this logic completely, but this is the minimum
change needed to fix the bug in v12.

Per report from Manuel Rigger.  Back-patch to v12 where the mistake
came in.

Discussion: https://postgr.es/m/CA+u7OA7nnGYy8rY0vdTe811NuA+Frr9nbcBO9u2Z+JxqNaud+g@mail.gmail.com

src/test/regress/expected/btree_index.out

@@ -198,6 +198,74 @@ select proname from pg_proc where proname ilike 'ri%foo' order by 1;
 reset enable_seqscan;
 reset enable_indexscan;
 reset enable_bitmapscan;
+-- Also check LIKE optimization with binary-compatible cases
+create temp table btree_bpchar (f1 text collate "C");
+create index on btree_bpchar(f1 bpchar_ops);
+insert into btree_bpchar values ('foo'), ('fool'), ('bar'), ('quux');
+-- doesn't match index:
+explain (costs off)
+select * from btree_bpchar where f1 like 'foo';
+          QUERY PLAN           
+-------------------------------
+ Seq Scan on btree_bpchar
+   Filter: (f1 ~~ 'foo'::text)
+(2 rows)
+
+select * from btree_bpchar where f1 like 'foo';
+ f1  
+-----
+ foo
+(1 row)
+
+explain (costs off)
+select * from btree_bpchar where f1 like 'foo%';
+           QUERY PLAN           
+--------------------------------
+ Seq Scan on btree_bpchar
+   Filter: (f1 ~~ 'foo%'::text)
+(2 rows)
+
+select * from btree_bpchar where f1 like 'foo%';
+  f1  
+------
+ foo
+ fool
+(2 rows)
+
+-- these do match the index:
+explain (costs off)
+select * from btree_bpchar where f1::bpchar like 'foo';
+                     QUERY PLAN                     
+----------------------------------------------------
+ Bitmap Heap Scan on btree_bpchar
+   Filter: ((f1)::bpchar ~~ 'foo'::text)
+   ->  Bitmap Index Scan on btree_bpchar_f1_idx
+         Index Cond: ((f1)::bpchar = 'foo'::bpchar)
+(4 rows)
+
+select * from btree_bpchar where f1::bpchar like 'foo';
+ f1  
+-----
+ foo
+(1 row)
+
+explain (costs off)
+select * from btree_bpchar where f1::bpchar like 'foo%';
+                                        QUERY PLAN                                        
+------------------------------------------------------------------------------------------
+ Bitmap Heap Scan on btree_bpchar
+   Filter: ((f1)::bpchar ~~ 'foo%'::text)
+   ->  Bitmap Index Scan on btree_bpchar_f1_idx
+         Index Cond: (((f1)::bpchar >= 'foo'::bpchar) AND ((f1)::bpchar < 'fop'::bpchar))
+(4 rows)
+
+select * from btree_bpchar where f1::bpchar like 'foo%';
+  f1  
+------
+ foo
+ fool
+(2 rows)
+
 --
 -- Test B-tree fast path (cache rightmost leaf page) optimization.
 --

src/test/regress/sql/btree_index.sql

@@ -83,6 +83,26 @@ reset enable_seqscan;
 reset enable_indexscan;
 reset enable_bitmapscan;
 
+-- Also check LIKE optimization with binary-compatible cases
+
+create temp table btree_bpchar (f1 text collate "C");
+create index on btree_bpchar(f1 bpchar_ops);
+insert into btree_bpchar values ('foo'), ('fool'), ('bar'), ('quux');
+-- doesn't match index:
+explain (costs off)
+select * from btree_bpchar where f1 like 'foo';
+select * from btree_bpchar where f1 like 'foo';
+explain (costs off)
+select * from btree_bpchar where f1 like 'foo%';
+select * from btree_bpchar where f1 like 'foo%';
+-- these do match the index:
+explain (costs off)
+select * from btree_bpchar where f1::bpchar like 'foo';
+select * from btree_bpchar where f1::bpchar like 'foo';
+explain (costs off)
+select * from btree_bpchar where f1::bpchar like 'foo%';
+select * from btree_bpchar where f1::bpchar like 'foo%';
+
 --
 -- Test B-tree fast path (cache rightmost leaf page) optimization.
 --