pgsql: pg_unicode_fast: fix final sigma logic.

pg_unicode_fast: fix final sigma logic.

If the string is preceded only by Case Ignorable characters, don't
consider it to be a final sigma.

In the process, refactor so that the preceding and following
characters are found first, and then the rule is applied, to improve
clarity.

Discussion: https://postgr.es/m/c355354e6c3f4a7aafb047361b73db247260fca0.camel@j-davis.com
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/36869368989ce37d85c08c33258eb01ae96e2375

Modified Files
--------------
src/common/unicode_case.c | 88 ++++++++++++++----------------
src/test/regress/expected/collate.utf8.out | 6 ++
src/test/regress/sql/collate.utf8.sql | 1 +
3 files changed, 47 insertions(+), 48 deletions(-)