Fix coerce_to_target_type for coerce_type's klugy handling of COLLATE.

Because coerce_type recurses into the argument of a CollateExpr,
coerce_to_target_type's longstanding code for detecting whether coerce_type
had actually done anything (to wit, returned a different node than it
passed in) was broken in 9.1.  This resulted in unexpected failures in
hide_coercion_node; which was not the latter's fault, since it's critical
that we never call it on anything that wasn't inserted by coerce_type.
(Else we might decide to "hide" a user-written function call.)

Fix by removing and replacing the CollateExpr in coerce_to_target_type
itself.  This is all pretty ugly but I don't immediately see a way to make
it nicer.

Per report from Jean-Yves F. Barbier.