i965: Add a SHADER_OPCODE_URB_READ_SIMD8_PER_SLOT opcode.

i965: Add a SHADER_OPCODE_URB_READ_SIMD8_PER_SLOT opcode.

We need to use per-slot offsets when there's non-uniform indexing,
as each SIMD channel could have a different index.  We want to use
them for any non-constant index (even if uniform), as it lives in
the message header instead of the descriptor, allowing us to set
offsets in GRFs rather than immediates.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>