nir: Add a function to count the number of vertices a GS emits.

nir: Add a function to count the number of vertices a GS emits.

Some hardware (such as Broadwell) can run geometry shaders more
efficiently when the number of vertices emitted is statically known.

This pass provides a way to obtain the constant vertex count, or
-1 indicating that the vertex count is unknown/non-constant.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>