Teach eval_const_expressions to constant-fold LEAST/GREATEST expressions.

Teach eval_const_expressions to constant-fold LEAST/GREATEST expressions.

Doing this requires an assumption that the invoked btree comparison
function is immutable.  We could check that explicitly, but in other
places such as contain_mutable_functions we just assume that it's true,
so we may as well do likewise here.  (If the comparison function's
behavior isn't immutable, the sort order in indexes built with it would
be unstable, so it seems certainly wrong for it not to be so.)

Vik Fearing

Discussion: https://postgr.es/m/c6e8504c-4c43-35fa-6c8f-3c0b80a912cc@2ndquadrant.com