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/shader_enums.h

@@ -48,6 +48,8 @@ typedef enum
    MESA_SHADER_COMPUTE = 5,
 } gl_shader_stage;
 
+typedef short gl_state_index16; /* see enum gl_state_index */
+
 const char *gl_shader_stage_name(gl_shader_stage stage);
 
 /**