pgsql: Fix LIKE/regex optimization for indexscan with exact-match patte

Fix LIKE/regex optimization for indexscan with exact-match pattern.

Commit 85b7efa1c introduced support for LIKE with non-deterministic
collations. By moving some conditionals around, it accidentally broke
the optimization for converting a LIKE or regex exact-match pattern
to an equality indexqual when the index collation doesn't match the
expression collation. That should be allowed if the expression
collation is deterministic. This patch re-introduces the optimization
for that common case.

One important beneficiary of this optimization is the "\d tablename"
command in psql. Without this fix that will do a seqscan on pg_class
instead of an index point lookup.

Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Author: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/DHBQIZX8SZVI.ZX614ZMFL645@jeltef.nl
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d0bb49e61168953f0f07c3beeb2b8fb63a3521a2

Modified Files
--------------
src/backend/utils/adt/like_support.c | 20 ++++++++++++++++----
src/test/regress/expected/collate.icu.utf8.out | 23 +++++++++++++++++++++++
src/test/regress/expected/collate.out | 23 +++++++++++++++++++++++
src/test/regress/sql/collate.icu.utf8.sql | 10 ++++++++++
src/test/regress/sql/collate.sql | 10 ++++++++++
5 files changed, 82 insertions(+), 4 deletions(-)