Should iterate over Program in wordSize()-steps when looking for
LABEL_OP and CHECK_HALT_OP instructions. Otherwise we run the risk of
misidentifying some part of an instruction which occupies more than one
slot in the program (such as BIT_VECTOR_OP, FORK_OP, JUMP_OP) as a
LABEL_OP or CHECK_HALT_OP. This wouldn't get us incorrect results, but
does unnecessarily drive up the size of MatchEnds, Seen, Live, and
CheckLabels.