compiler: Extract num_mesh_vertices_per_primitive function.

Prevent code duplication.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15005>

src/compiler/shader_enums.h

@@ -1045,6 +1045,11 @@ enum shader_prim
    SHADER_PRIM_UNKNOWN = (SHADER_PRIM_MAX * 2),
 };
 
+/**
+ * Number of vertices per mesh shader primitive.
+ */
+unsigned num_mesh_vertices_per_primitive(unsigned prim);
+
 /**
  * A compare function enum for use in compiler lowering passes.  This is in
  * the same order as GL's compare functions (shifted down by GL_NEVER), and is