compiler: Add SYSTEM_VALUE_IS_INDEXED_DRAW and instrinsics

This VS system value contains if the draw command used to start the
rendering was an indexed draw command or a non-indexed one
(~0/0 respectively). Useful to calculate the gl_BaseVertex as:
(SYSTEM_VALUE_IS_INDEXED_DRAW & SYSTEM_VALUE_FIRST_VERTEX).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/shader_enums.h

@@ -517,6 +517,13 @@ typedef enum
     */
    SYSTEM_VALUE_FIRST_VERTEX,
 
+   /**
+    * If the Draw command used to start the rendering was an indexed draw
+    * or not (~0/0). Useful to calculate \c SYSTEM_VALUE_BASE_VERTEX as
+    * \c SYSTEM_VALUE_IS_INDEXED_DRAW & \c SYSTEM_VALUE_FIRST_VERTEX.
+    */
+   SYSTEM_VALUE_IS_INDEXED_DRAW,
+
    /**
     * Value of \c baseinstance passed to instanced draw entry points
     *