i965: Introduce new SHADER_OPCODE_URB_WRITE_SIMD8_MASKED/PER_SLOT opcodes.

i965: Introduce new SHADER_OPCODE_URB_WRITE_SIMD8_MASKED/PER_SLOT opcodes.

In the vec4 backend, we have a vec4_instruction::urb_write_flags field.
There are many kinds of flags for SIMD4x2 messages.

However, there are really only two (per-slot offset, use channel masks)
for SIMD8 messages.  Rather than adding a boolean flag for per-slot
offsets (polluting all instructions), I decided to just make three new
opcodes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>