Restore a FIXME that someone should not have been deleted; add a new one.

The first comment is not mine. It was rightly added in d57319a5
("FASL changes for 64-bit compatibility") but Nikodemus removed it,
having fixed only one of the weird declarations. There are 6 others.
Make DEFINE-FOP more declarative and perform fewer underflow checks.
In FOP-FUNCALL{-FOR-EFFECT} check for stack underflow once only.
Tweak LOAD-FASL-GROUP