intel/compiler/fs: Add support for 16-bit sampler msg payload

intel/compiler/fs: Add support for 16-bit sampler msg payload

For SIMD8 half float payload, each component takes a full register, so
we can use existing LOAD_PAYLOAD infrastruture for required padding by
alternating plain 8-wide half float vector and null vector.

Also this patch removes an unwanted assertion from
opt_copy_propagation_local for LOAD_PAYLOAD.

Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11766>