i965: Add optimization pass to let us use the replicate data message

i965: Add optimization pass to let us use the replicate data message

The data port has a SIMD16 'replicate data' message, which lets us write
the same color for all 16 pixels by sending the four floats in the
lower half of a register instead of sending 4 times 16 identical
component values in 8 registers.

The message comes with a lot of restrictions and could be made generally
useful by recognizing when those restriction are satisfied.  For now,
this lets us enable the optimization when we know it's safe, but we don't
enable it by default.  The optimization works for simple color clear shaders
only, but does recognized and support multiple render targets.

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