Whole structures are assignable

Whole arrays are assignable in GLSL 1.20 and later, but it's not clear
how to handle that within the IR because the IR is supposed to be
shading language version agnostic.

ir.cpp

@@ -200,8 +200,11 @@ ir_dereference::is_lvalue()
    if (var == NULL)
       return false;
 
-   if (this->type->base_type == GLSL_TYPE_ARRAY ||
-       this->type->base_type == GLSL_TYPE_STRUCT)
+   /* Arrays are not assignable in GLSL 1.10, but in GLSL 1.20 and later they
+    * are.
+    */
+   /* FINISHME: Handle GLSL 1.10 vs 1.20 differences. */
+   if (this->type->base_type == GLSL_TYPE_ARRAY)
       return false;
 
    if (mode == ir_reference_variable) {