[IMP] web: do not eval modifiers if not necessary

Before this commit, the web client evaluated all modifiers
(readonly/invisible/required) in some cases, even if it only needed one
of them.  This is clearly not optimal, and I even suppose that for large o2m,
it could even have a noticeable effect on web client perceived speed

This commit simply make sure that we evaluate only what we need in those
cases.  Note that it is not really a testable improvement, hence the
lack of tests.

closes odoo/odoo#29171