pgsql: Fix corner-case failure in match_pattern_prefix().

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3c265d7be42484bd0ab4a9c0a920289e8f5c995

Modified Files
--------------
src/backend/utils/adt/like_support.c      | 11 +++--
src/test/regress/expected/btree_index.out | 68 +++++++++++++++++++++++++++++++
src/test/regress/sql/btree_index.sql      | 20 +++++++++
3 files changed, 95 insertions(+), 4 deletions(-)