ART: Added dirty flag to optimizations

- Added a dirty flag to the PassMEDataHolder and use it to determine
    if we want to perform the post-optimization passes or not.

Change-Id: I59a85f5198f9224d5fe8893397bfaf68a978d8f1
Signed-off-by: Jean Christophe Beyler <jean.christophe.beyler@intel.com>