i965/fs: Reduce the response length of sampler messages on Skylake.

i965/fs: Reduce the response length of sampler messages on Skylake.

Often, we don't need a full 4 channels worth of data from the sampler.
For example, depth comparisons and red textures only return one value.
To handle this, the sampler message header contains a mask which can
be used to disable channels, and reduce the message length (in SIMD16
mode on all hardware, and SIMD8 mode on Broadwell and later).

We've never used it before, since it required setting up a message
header.  This meant trading a smaller response length for a larger
message length and additional MOVs to set it up.

However, Skylake introduces a terrific new feature: for headerless
messages, you can simply reduce the response length, and it makes
the implicit header contain an appropriate mask.  So to read only
RG, you would simply set the message length to 2 or 4 (SIMD8/16).

This means we can finally take advantage of this at no cost.

total instructions in shared programs: 9091831 -> 9073067 (-0.21%)
instructions in affected programs: 191370 -> 172606 (-9.81%)
helped: 2609
HURT: 0

total cycles in shared programs: 70868114 -> 68454752 (-3.41%)
cycles in affected programs: 35841154 -> 33427792 (-6.73%)
helped: 16357
HURT: 8188

total spills in shared programs: 3492 -> 1707 (-51.12%)
spills in affected programs: 2749 -> 964 (-64.93%)
helped: 74
HURT: 0

total fills in shared programs: 4266 -> 2647 (-37.95%)
fills in affected programs: 3029 -> 1410 (-53.45%)
helped: 74
HURT: 0

LOST:   1
GAINED: 143

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>