intel/compiler: Use SIMD16 instructions in fs saturate prop unit test

intel/compiler: Use SIMD16 instructions in fs saturate prop unit test

Will allow us to test that propagation between instructions of different
exec sizes does not happen (in the next commit).

The stray-looking change in intervening_dest_write is to adjust the size
of the texture result to keep the test functioning identically when the
instructions' exec sizes are doubled. Without the change, the texture
does not overwrite the destination fully as the unit test intends.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>