Fix LIKE's special-case code for % followed by _. I'm not entirely sure that

this case is worth a special code path, but a special code path that gets
the boundary condition wrong is definitely no good.  Per bug #4821 from
Andrew Gierth.

In passing, clean up some minor code formatting issues (excess parentheses
and blank lines in odd places).

Back-patch to 8.3, where the bug was introduced.