Avoid PR72749 by not using unspecs

[Alan Modra]

On Sat, Jan 14, 2017 at 03:51:27AM -0600, Segher Boessenkool wrote:
> It also prevents combine from combining any instruction into an existing
> doloop insn (resulting in a doloop insn again).  This isn't too serious,
> almost always this is just a register copy that will be optimised away
> some other way.

Right.  I forgot to mention, I looked at all of gcc/*.o before/after
the patch and saw no occurrences of any missed combines.  Just the
expected changes in rs6000.o, various insn-*.o from twiddling
rs6000.md, and one constant change in read-rtl-function.o.

-- 
Alan Modra
Australia Development Lab, IBM