intel/fs: Move packHalf2x16 handling to lower_pack()

intel/fs: Move packHalf2x16 handling to lower_pack()

This mainly lets the software scoreboarding pass correctly mark the
instructions, without needing to resort to fragile manual handling in
the generator.

We can also make small improvements.  On Gfx 8LP-12.0, we no longer have
the restrictions about DWord alignment, so we can simply write each half
into its intended location, rather than writing it to the low DWord and
then shifting it in place.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21783>