intel/compiler: fix ddx and ddy for 16-bit float

intel/compiler: fix ddx and ddy for 16-bit float

We were assuming 32-bit elements. Also, In SIMD8 we pack 2 vector components
in a single SIMD register, so for example, component Y of a 16-bit vec2
starts is at byte offset 16B. This means that when we compute the offset of
the elements to be differentiated we should not stomp whatever base offset we
have, but instead add to it.

v2
 - Use byte_offset() helper (Jason)
 - Merge the fix for SIMD8: using byte_offset() fixes that too.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> (v1)
Reviewed-by: Matt Turner <mattst88@gmail.com>