i965/fs: Use untyped_surface_read for 16-bit load_ssbo

i965/fs: Use untyped_surface_read for 16-bit load_ssbo

SSBO loads were using byte_scattered read messages as they allow
reading 16-bit size components. byte_scattered messages can only
operate one component at a time so we needed to emit as many messages
as components.

But for vec2 and vec4 of 16-bit, being multiple of 32-bit we can use the
untyped_surface_read message to read pairs of 16-bit components using only
one message. Once each pair is read it is unshuffled to return the proper
16-bit components. vec3 case is assimilated to vec4 but the 4th component
is ignored.

16-bit scalars are read using one byte_scattered_read message.

v2: Removed use of stride = 2 on sources (Jason Ekstrand)
    Rework optimization using unshuffle 16 reads (Chema Casanova)
v3: Use W and D types insead of HF and F in shuffle to avoid rounding
    erros (Jason Ekstrand)
    Use untyped_surface_read for 16-bit vec3. (Jason Ekstrand)
v4: Use subscript insead of chaging type and stride  (Jason Ekstrand)

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