The current implementation of control by checking on x86 has an unnecessary sign extension after the doing the control byte comparison. Changing the bitmask object
 to explicitly track only 16 bits (instead of 32) eliminates this, saving an instruction / cycle. This speeds up hit checking by up to 6% on Milan and up to 15% on CLX

PiperOrigin-RevId: 572965182
Change-Id: Ifda0e3250d409266d6dcef89cba6ada91d879291