i965/fs: Use byte scattered read for 16-bit load_ssbo

i965/fs: Use byte scattered read for 16-bit load_ssbo

Used to enable 16-bit reads at do_untyped_vector_read, that is used on
the following intrinsics:

   * nir_intrinsic_load_shared
   * nir_intrinsic_load_ssbo

v2: Removed use of stride = 2 on 16-bit sources (Jason Ekstrand)

v3: - Add bitsize to scattered read operation (Jason Ekstrand)
    - Remove implementation of 16-bit UBO read from this patch.
    - Avoid assertion at opt_algebraic caused by ADD of two IMM with
      offset with BRW_REGISTER_TYPE_UD type found on matrix tests.
      (Jose Maria Casanova)
v4: (Jason Ekstrand)
    - Put if case for 16-bits at the beginning of the if ladder.
    - Use type_sz(dest.type) * 8 as bit_size parameter for scattered read.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>