i965/simd8vs: Fix SIMD8 atomics

i965/simd8vs: Fix SIMD8 atomics

The short version: we need to set bits in R0.7 which provide a mask to be used
for PS kill samples/pixels. Since the VS has no such concept, we just need to
set all 1.

The longer version...
Execution for SIMD8 atomics is defined as follows:
SIMD8: The low 8 bits of the execution mask are ANDed with 8 bits of the
Pixel/Sample Mask from the message header. For the typed messages, the Slot
Group in the message descriptor selects either the low or high 8 bits. For the
untyped messages, the low 8 bits are always selected. The resulting mask is used
to determine which slots are read into the destination GRF register (for read),
or which slots are written to the surface (for write). If the header is not
present, only the low 8 bits of the execution mask are used.

The message header for untyped messages is defined in R0.7 "This field contains
the 16-bit pixel/sample mask to be used for SIMD16 and SIMD8 messages. All 16
bits are used for SIMD16 messages.  For typed SIMD8 messages, Slot Group selects
which 8 bits of this field are used. For untyped SIMD8 messages, the low 8 bits
of this field are used." Furthermore, "The message header for the untyped
messages only needs to be delivered for pixel shader threads, where the
execution mask may indicate pixels/samples that are enabled only due to
derivative (LOD) calculations, but the corresponding slot on the surface must
not be accessed." We're not using a pixel shader here, but AFAICT, this mask is
used for all stages.

This leaves two options, Remove the header, or make the VS code emit the correct
thing for the header. I believe one of the goals of using SIMD8 VS was to get as
much code reuse as possible, and so I chose the latter. Since the VS has no such
thing as kill instructions, the mask is derived simple as all 1's.

v2:
Add a comment to the code (stolen from Curro on the mailing list)
Change the control flow style (Curro + Jason)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87258
Cc: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>