nir: Add new GS intrinsics that maintain a count of emitted vertices.

This patch also introduces a lowering pass to convert the simple GS
intrinsics to the new ones.  See the comments above that for the
rationale behind the new intrinsics.

This should be useful for i965; it's a generic enough mechanism that I
could see other drivers potentially using it as well, so I don't feel
too bad about putting it in the generic code.

v2:
- Use nir_after_block_before_jump for the cursor (caught by Jason
  Ekstrand - I'd mistakenly used nir_after_block when rebasing this
  code onto the new NIR control flow API).
- Remove the old emit_vertex intrinsic at the end, rather than in
  the middle (requested by Jason).
- Use state->... directly rather than locals (requested by Jason).
- Report progress from nir_lower_gs_intrinsics() (requested by me).
- Remove "Authors:" section from file comment (requested by
  Michael Schellenberger Costa).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/glsl/nir/nir_intrinsics.h

@@ -79,9 +79,30 @@ BARRIER(memory_barrier)
 /** A conditional discard, with a single boolean source. */
 INTRINSIC(discard_if, 1, ARR(1), false, 0, 0, 0, 0)
 
+/**
+ * Basic Geometry Shader intrinsics.
+ *
+ * emit_vertex implements GLSL's EmitStreamVertex() built-in.  It takes a single
+ * index, which is the stream ID to write to.
+ *
+ * end_primitive implements GLSL's EndPrimitive() built-in.
+ */
 INTRINSIC(emit_vertex,   0, ARR(), false, 0, 0, 1, 0)
 INTRINSIC(end_primitive, 0, ARR(), false, 0, 0, 1, 0)
 
+/**
+ * Geometry Shader intrinsics with a vertex count.
+ *
+ * Alternatively, drivers may implement these intrinsics, and use
+ * nir_lower_gs_intrinsics() to convert from the basic intrinsics.
+ *
+ * These maintain a count of the number of vertices emitted, as an additional
+ * unsigned integer source.
+ */
+INTRINSIC(emit_vertex_with_counter, 1, ARR(1), false, 0, 0, 1, 0)
+INTRINSIC(end_primitive_with_counter, 1, ARR(1), false, 0, 0, 1, 0)
+INTRINSIC(set_vertex_count, 1, ARR(1), false, 0, 0, 0, 0)
+
 /*
  * Atomic counters
  *