Implement -Wswitch-fallthrough: rs6000

[Bruce Korb]

> On Mon, Jul 11, 2016 at 01:36:02PM -0700, Bruce Korb wrote:
> [[putrid code deleted]]
>> Does this patch mean that the above got fixed?  I mean, if you're
>> going to fret over linguistic tags to make falling through explicit,
>> it would seem the above code is pretty sore-thumby, yes?
>
> My current implementation warns here, but the warning can be suppressed
> by adding /* FALLTHRU */ or __builtin_fallthrough(); before the
> do_something_else() line.
>
> Does that answer your question?

Not in a satisfying way. :)  I understand that syntactically, you can
drop a "case" in wherever you can drop a statement label.  That
doesn't mean you should.  Since I wasn't fixing GCC code, I just
rolled my eyes and moved on.  However, if the code is going to be
changed, then contortions like that ought to be addressed.  Both for
aesthetics and for code clarity.