i965/gs: Refactor ir_emit_vertex and ir_end_primitive

i965/gs: Refactor ir_emit_vertex and ir_end_primitive

So the implementation is independent of GLSL IR and the visit methods of the
vec4 visitor. This way we will be able to reuse that implementation directly
from the NIR vec4 backend.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>