glsl/shader_enums: Add an enum for Vulkan InstanceIndex

In Vulkan, you have InstanceIndex which begins at the base instance value
rather than the zero-based InstanceID of GL.

Reviewed-by: Rob Clark <robdclark@gmail.com>

src/compiler/shader_enums.c

@@ -201,6 +201,7 @@ gl_system_value_name(gl_system_value sysval)
    static const char *names[] = {
      ENUM(SYSTEM_VALUE_VERTEX_ID),
      ENUM(SYSTEM_VALUE_INSTANCE_ID),
+     ENUM(SYSTEM_VALUE_INSTANCE_INDEX),
      ENUM(SYSTEM_VALUE_VERTEX_ID_ZERO_BASE),
      ENUM(SYSTEM_VALUE_BASE_VERTEX),
      ENUM(SYSTEM_VALUE_INVOCATION_ID),

src/compiler/shader_enums.h

@@ -378,6 +378,13 @@ typedef enum
     */
    SYSTEM_VALUE_INSTANCE_ID,
 
+   /**
+    * Vulkan InstanceIndex.
+    *
+    * InstanceIndex = gl_InstanceID + gl_BaseInstance
+    */
+   SYSTEM_VALUE_INSTANCE_INDEX,
+
    /**
     * DirectX-style vertex ID.
     *