havingQual vs hasHavingQual buglets

I came across a couple of places in the planner that are checking
for nonempty havingQual; but since these bits run after
const-simplification of the HAVING clause, that produces the wrong
answer for a constant-true HAVING clause (which'll be folded to
empty). Correct code is to check root->hasHavingQual instead.

These mistakes only affect cost estimates, and they're sufficiently
corner cases that it'd be hard even to devise a reliable test case
showing a different plan choice. So I'm not very excited about this,
and am thinking of committing only to HEAD.

regards, tom lane