i965/gs: Optimize away the EOT write on Gen8+ with static vertex count.

i965/gs: Optimize away the EOT write on Gen8+ with static vertex count.

With static vertex counts, the final EOT write doesn't actually write
any data - it's just there to end the thread.  Typically, the last
thing before ending the thread will be an EmitVertex() call, resulting
in a URB write.  We can just set EOT on that.

Note that this isn't always possible - there might be an intervening
SSBO write/image store, or the URB write may have been in a loop.

shader-db statistics for geometry shaders only:

total instructions in shared programs: 3173 -> 3149 (-0.76%)
instructions in affected programs:     176 -> 152 (-13.64%)
helped:                                8

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