Adjust constant-folding of CASE expressions so that the simple comparison

form of CASE (eg, CASE 0 WHEN 1 THEN ...) can be constant-folded as it
was in 7.4.  Also, avoid constant-folding result expressions that are
certainly unreachable --- the former coding was a bit cavalier about this
and could generate unexpected results for all-constant CASE expressions.
Add regression test cases.  Per report from Vlad Marchenko.