Cache result to speed up IRDB GetIDAOpcode().

include/interfaces/abstract/STARSInstruction.h

include/interfaces/idapro/STARSInstruction.h

include/interfaces/irdb/STARSInstruction.h

include/interfaces/irdb/STARSSSAInstruction.h

src/interfaces/abstract/STARSInstruction.cpp

src/interfaces/abstract/STARSOp.cpp

src/interfaces/idapro/STARSIDAInstruction.cpp

src/interfaces/irdb/STARS_IRDB_Instruction.cpp