Fix Stmt:Data object leak

call Stmt::destroyData() whenever a Stmt object is to be removed in
BasicBlock to delete the Stmt::Data object hold in the Stmt object.

Change-Id: I59c939d79b935153e6f8613e54f149120f5198f5
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>