i965/fs: Add empirically-determined instruction latencies for gen7.

i965/fs: Add empirically-determined instruction latencies for gen7.

v2: Actually switch on the other math instructions mentioned in the
    comment.
v3: Add timing data for textureSize(), and clean up some long comment
    lines.

Testing shader_time of fs16 shaders on a few frames of various apps:
nexuiz improved by 2.9% +/- 1.5% (n=10)
no difference on GLB2.5 (n=36, outliers removed)
no difference on GLB2.7 (n=25)
etqw improved by 2.6% +/- 2.2% (n=25)
no difference on lightsmark (n=25)

Acked-by: Kenneth Graunke <kenneth@whitecape.org>