mesa,glsl,nir: reduce gl_state_index size to 2 bytes

Let's use the new gl_state_index16 type everywhere and remove
the typecasts.

This helps reduce the size of gl_program_parameter.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/compiler/glsl/ir.h

@@ -397,7 +397,7 @@ depth_layout_string(ir_depth_layout layout);
  * \sa ir_variable::state_slots
  */
 struct ir_state_slot {
-   int tokens[5];
+   gl_state_index16 tokens[5];
    int swizzle;
 };