Make read-only variables not be considered lvalues.

This should fix tests trying to assign to builtin "in" variables, and
will also be relied on for post_inc/dec handling.

ir.h

@@ -455,7 +455,17 @@ public:
 
    bool is_lvalue()
    {
-      return var != NULL;
+      ir_variable *as_var;
+
+      if (var == NULL)
+	 return NULL;
+
+      as_var = var->as_variable();
+
+      if (as_var == NULL)
+	 return NULL;
+
+      return !as_var->read_only;
    }
 
    enum {