[RFA:] In -build-operand!, -build-reg-operand, collect the natural mode, not the used mode,

[Doug Evans]

Doug Evans writes:
 > Ugh, I just realized that this patch doesn't handle floating point.
 > And I've left as a todo the checking of legal mode conversions.

And I don't properly handle sets.
I was going to, but it was 3 in the morning. :-)

I think what we want to do is require all sets of registers
to use the natural mode of the register.
For registers like x86's al,ah,ax,eax, we can either
- add set-partial, or some such
- use (set (subword ...) ...)
- require the description of additional registers that are
  smaller-mode aliases of the "real" register

I think I prefer either of the latter two.