Now that we're using 32-bit ops for 8/16-bit logical operations
(to avoid partial register stalls), there's really no need to
keep track of the 8/16-bit ops at all.  Convert everything we
can to 32-bit ops.

This CL is the obvious stuff.  I might think a bit more about
whether we can get rid of weirder stuff like HMULWU.

The only downside to this CL is that we lose some information
about constants.  If we had source like:
  var a byte = ...
  a += 128
  a += 128
We will convert that to a += 256, when we could get rid of the
add altogether.  This seems like a fairly unusual scenario and
I'm happy with forgoing that optimization.

Change-Id: Ia7c1e5203d0d110807da69ed646535194a3efba1
Reviewed-on: https://go-review.googlesource.com/22382Reviewed-by: Todd Neal <todd@tneal.org>