[assert]: remaining c*.c files

[Giovanni Bajo]

> yeah, I'm not entirely sure what the best solution is here.  I've come
> across some other instances in emit-rtl.c.  gcc_unreachable is not
> quite right, because, if the condition is met, that point will be reached.
> (If we're telling the compiler that that point is unreachable, when it
becomes
> sufficiently smart, it'll deduce that all paths to that point are
untraversable
> ...)
>
> We should probably be explicitly saying 'internal_error'.  puking the
cgraph
> node looks kind of unprofessional in a release compiler.

I see. Yes, maybe aborting with internal_error() + the cgraph dump iff
ENABLE_CHECKING is the best solution.

Giovanni Bajo