Fix "element <@ range" cost estimation.

The statistics-based cost estimation patch for range types broke that, by
incorrectly assuming that the left operand of all range oeprators is a
range. That lead to a "type x is not a range type" error. Because it took so
long for anyone to notice, add a regression test for that case.

We still don't do proper statistics-based cost estimation for that, so you
just get a default constant estimate. We should look into implementing that,
but this patch at least fixes the regression.

Spotted by Tom Lane, when testing query from Josh Berkus.