Re: BUG #17570: Unrecognized node type for query with statistics on expressions

* statext_is_compatible_clause_internal is not on board with
offsetting the attnums.

* Neither is this check for a whole-row reference (line 1628, in HEAD):

                if (bms_is_member(InvalidAttrNumber, clause_attnums))

* Although the coverage report claims this code is exercised, the
fact that nothing failed when you made only a partial fix says it's
not exercised well enough.

* The comments for statext_is_compatible_clause suck.  If they'd
defined what the arguments are, perhaps this mess would have been
prevented.  For extra credit, it'd be really nice to define what
"is compatible" means.  I'd sure not have thought that that would
include permissions checks.