Normally, we expect SIMD8 shaders to be more instructions than SIMD4x2
shaders, as it takes four instructions to operate on a vec4, rather than
a single instruction.  However, the benefit is that it can process 8
objects per shader thread instead of 2.

Surprisingly, the shader-db statistics show an improvement in both
instruction and cycle counts:

Synmark: -31.25% instructions, -29.27% cycles, 0 hurt.
Tessmark: -36.92% instructions, -37.81% cycles, 0 hurt.
Unigine Heaven: -3.42% instructions, -17.95% cycles, 0 hurt.
Shadow of Mordor:
   +13.24% instructions (26 with fewer instructions, 45 with more),
   -5.23% cycles (44 with fewer cycles, 27 with more cycles).

Presumably, this is because the SIMD8 URB messages are a much more
natural fit than the SIMD4x2 URB messages - there's a ton less header
setup.

I benchmarked Shadow of Mordor and Unigine Heaven on my Skylake GT3e,
and the performance seems to be the same or increase ever so slightly
(< 1 FPS difference).  So I believe it's strictly superior.

There's also a lot more optimization potential we can do in scalar mode.

This will also help us finish fp64 support, as scalar support is going
to land much sooner than vec4-mode support.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>