intel/fs: Simplify fs_visitor::emit_samplepos_setup

intel/fs: Simplify fs_visitor::emit_samplepos_setup

The original code manually handled splitting the MOVs to 8-wide to
handle various regioning restrictions.  Now that we have a SIMD width
splitting pass that handles these things, we can just emit everything at
the full width and let the SIMD splitting pass handle it.  We also now
have a useful "subscript" helper which is designed exactly for the case
where you want to take a W type and read it as a vector of Bs so we may
as well use that too.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>