Minor regression due to recent IRA changes

[Oleg Endo]

On Sat, 2020-02-29 at 09:38 -0700, Jeff Law wrote:
> 
> It really would have just been a workaround for some of the R0 issues anyway. 
> I think at its core R0 on the SH probably needs to be treated more like a
> temporary rather than a general register.  But that's probably a huge change,
> both in terms of just getting it working right and in terms of addressing the
> code quality regressions that would introduce.
> 

I think one of the major issues is that R0 is a constraint in several
addressing modes for memory accesses.  I believe I once had the idea of
hiding R0 from RA ... then insert reg-reg copies (to load R0) after
RA/reload ... and then somehow do back propagation to get rid of the
reg-reg copies again.  Another idea was to run a pre-RA pass to pre-
allocate all R0 things.  But I think it's all just running in sqrt(1)
circles after all.

Cheers,
Oleg