i965/fs_nir: Get rid of get_alu_src

i965/fs_nir: Get rid of get_alu_src

Originally, get_alu_src was supposed to handle resolving swizzles and
things like that.  However, now that basically every instruction we have
only takes scalar sources, we don't really need it anymore.  The only case
where it's still marginally useful is for the mov and vecN operations that
are left over from SSA form.  We can handle those cases as a special case
easily enough.  As a side-effect, we don't need the vec_to_movs pass
anymore.

v2 Jason Ekstrand <jason.ekstrand@intel.com>:
 - Rework the way we detect if we need an extra copy for swizzling.  The
   old code involved a pile of confusing switch fall-throughs; we now use a
   loop.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>