i965/fs: Support 16-bit store_ssbo with VK_KHR_relaxed_block_layout

i965/fs: Support 16-bit store_ssbo with VK_KHR_relaxed_block_layout

Restrict the use of untyped_surface_write with 16-bit pairs in
ssbo to the cases where we can guarantee that offset is multiple
of 4.

Taking into account that VK_KHR_relaxed_block_layout is available
in ANV we can only guarantee that when we have a constant offset
that is multiple of 4. For non constant offsets we will always use
byte_scattered_write.

v2: (Jason Ekstrand)
    - Assert offset_reg to be multiple of 4 if it is immediate.

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