glsl: introduce data section to ir_variable

Data section helps serialization and cloning of a ir_variable. This
patch includes the helper bits used for read only ir_variables.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/glsl/loop_analysis.h

@@ -217,7 +217,8 @@ public:
 
       /* Variables that are marked read-only *MUST* be loop constant.
        */
-      assert(!this->var->read_only || (this->var->read_only && is_const));
+      assert(!this->var->data.read_only
+            || (this->var->data.read_only && is_const));
 
       return is_const;
    }