broadcom/compiler: don't favor/select accum registers for hw not supporting it

Note that what we do is to just return false on the favor/select accum
methods. We could just avoid to call them, but as the select is called
more than once, it is just easier this way.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25450>