glsl: Add a predicate to determine whether a variable is an interface block

For the first declaration below, there will be an ir_variable named
"instance" whose type and whose instance_type will be the same
glsl_type.  For the second declaration, there will be an ir_variable
named "f" whose type is float and whose instance_type is B2.

"instance" is an interface instance variable, but "f" is not.

uniform B1 {
    float f;
} instance;

uniform B2 {
    float f;
};

v2: Copy the comment message documentation into the code.  Suggested by
Paul Berry.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ir.h

@@ -357,6 +357,33 @@ public:
       return this->mode == ir_var_uniform && this->interface_type != NULL;
    }
 
+   /**
+    * Determine whether or not a variable is the declaration of an interface
+    * block
+    *
+    * For the first declaration below, there will be an \c ir_variable named
+    * "instance" whose type and whose instance_type will be the same
+    *  \cglsl_type.  For the second declaration, there will be an \c ir_variable
+    * named "f" whose type is float and whose instance_type is B2.
+    *
+    * "instance" is an interface instance variable, but "f" is not.
+    *
+    * uniform B1 {
+    *     float f;
+    * } instance;
+    *
+    * uniform B2 {
+    *     float f;
+    * };
+    */
+   inline bool is_interface_instance() const
+   {
+      const glsl_type *const t = this->type;
+
+      return (t == this->interface_type)
+         || (t->is_array() && t->fields.array == this->interface_type);
+    }
+
    /**
     * Declared type of the variable
     */