Optimize constant MinMax expressions

I was working on a little thing where I needed to simulate BETWEEN
SYMMETRIC so naturally I used least() and greatest().  I was a little
surprised to see that my expressions were not folded into straight
constants and the estimates were way off as a consequence.

I came up with the attached patch to fix it, but it's so ridiculously
small that I fear I'm missing something.

I don't think this needs any documentation and I didn't see where we
have any existing tests for eval_const_expressions so I didn't create
any either.

Thoughts?
-- 
Vik Fearing                                          +33 6 46 75 15 36
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support