ART: Add a last item in the MIROptimizationFlagPositions enumeration

- Added a last item in the enumeration in order to be able to notice
  when an extended MIR opcode is beyond the last one known to libart, and hence
  must be an opcode added by code from a plugin.
- Fixed the naming typo of the enumeration.

Change-Id: I0e021ba54b0e60531338f23ca0ab64755e15229b
Signed-Off-By: Jean Christophe Beyler <jean.christophe.beyler@intel.com>