Make decompilation of optimized CASE constructs more robust.

Make decompilation of optimized CASE constructs more robust.

We had some hacks in ruleutils.c to cope with various odd transformations
that the optimizer could do on a CASE foo WHEN "CaseTestExpr = RHS" clause.
However, the fundamental impossibility of covering all cases was exposed
by Heikki, who pointed out that the "=" operator could get replaced by an
inlined SQL function, which could contain nearly anything at all.  So give
up on the hacks and just print the expression as-is if we fail to recognize
it as "CaseTestExpr = RHS".  (We must cover that case so that decompiled
rules print correctly; but we are not under any obligation to make EXPLAIN
output be 100% valid SQL in all cases, and already could not do so in some
other cases.)  This approach requires that we have some printable
representation of the CaseTestExpr node type; I used "CASE_TEST_EXPR".

Back-patch to all supported branches, since the problem case fails in all.