SH: use register names instead of numbers

[Joern Rennecke]

> It would be easier to implement the other way around - how about if
> one of the generator programs writes out #defines for each constant?
> insn-codes.h or insn-config.h seem sensible places. Or we could have a
> separate genconstants program, if none of the existing insn-*.h files
> fit.

Sounds like a good plan.

If I write a patch for this, is someone willing to review it?