Add a partial_subreg_p predicate

[Jeff Law]

On 08/30/2017 09:41 AM, Richard Sandiford wrote:
> 
> Even then I don't think we could ever have SET_SRC and SET_DEST being
> different.  The extension has to be explicit in the source.
Yea, you should still have the same mode for the SET_SRC and SET_DEST.
It's just that the operands underneath may have different modes.

ie

(set (reg:SI) (zero_extend:SI (reg:HI))

So the SET_SRC/SET_DEST have the same modes, but the operands presented
to the allocators and reloaders have different modes.

The only reason I remember this being an "interesting" case is because
in the old allocator (pre-IRA) we were not good at tying objects of
different sizes.

Jeff