Fix segfault on degenerate bitfield case

[Richard Sandiford]

Eric Botcazou <ebotcazou@adacore.com> writes:
> This is a segfault on a degenerate bitfield case introduced by the rewrite of 
> the bitfield machinery.  In Ada, we have bitfields of size zero and we ask the 
> middle-end to generate accesses to them.  This doesn't work anymore because 
> get_best_mode now returns VOIDmode instead of QImode in this case, which 
> wreaks havoc later.
>
> The patchlet just restores the previous behaviour.  It also makes the comment 
> describing the computation of bitregion_end_ more explicit, as the original 
> formulation is a bit terse on second reading, even for the reviewer. :-)
>
> Bootstrapped/regtested on x86-64/Linux, applied on the mainline as obvious.

Thanks!

Richard