Fix PR44281 (bad RA with global regs)

[Jeff Law]

On 02/26/2016 08:41 AM, Bernd Schmidt wrote:
> On 02/22/2016 03:37 PM, Richard Biener wrote:
>
>> Do calls properly clobber them even if they are not in the set of
>> call-clobbered regs?
>> Are asm()s properly using/clobbering them?  I think you are allowed to
>> use them
>> in asm()s without adding constraints for them?
>
> Calls do, asms currently don't AFAICT. Not sure whether it's allowed to
> use them, but I think it should be straightforward to adjust df-scan.
The other case that came to mind was signal handlers.  What happens if 
we're using the global register as a scratch, we hit a memory reference 
that faults and inside the signal handler the original code expects to 
be able to use the global register the user set up?

If that's a valid use scenario, then there's probably all kinds of DF 
stuff that we'd need to expose.

jeff