Separate out psql_completion's giant else-if chain of *Matches
tests into a new function. Add the infrastructure needed for
table-driven checking of the initial match of each completion
rule. As-is, however, the code continues to operate as it did.
The new behavior applies only if SWITCH_CONVERSION_APPLIED
is #defined, which it is not here. (The preprocessor added
in the next patch will add a #define for that.)
The first and last couple of bits of psql_completion are not
based on HeadMatches/TailMatches/Matches tests, so they stay
where they are; they won't become part of the switch.
This patch also fixes up a couple of if-conditions that didn't meet
the conditions enumerated in the comment for match_previous_words().
Those restrictions exist to simplify the preprocessor.
Discussion: https://postgr.es/m/2208466.1720729502@sss.pgh.pa.us
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