Prevent bogus pullup of constant-valued functions returning composite.

Prevent bogus pullup of constant-valued functions returning composite.

Fix an oversight in commit 7266d0997: as it stood, the code failed
when a function-in-FROM returns composite and can be simplified
to a composite constant.

For the moment, just test for composite result and abandon pullup
if we see one.  To make it actually work, we'd have to decompose
the composite constant into per-column constants; which is surely
do-able, but I'm not convinced it's worth the code space.

Per report from Raúl Marín Rodríguez.

Discussion: https://postgr.es/m/CAM6_UM4isP+buRA5sWodO_MUEgutms-KDfnkwGmryc5DGj9XuQ@mail.gmail.com