intel/compiler: Delete key->msaa_16

intel/compiler: Delete key->msaa_16

None of the drivers have used this since we dropped i965, and BLORP
no longer uses it as of the previous commit.  We can also drop the
former compressed_multisample_tex_mask (now padding) field so that
things remain 64-bit aligned.

Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20223>