op_reg_common.h: Refactor variable for safety

op_reg_common.h: Refactor variable for safety

This patch changes the variable that tells how many common bits there
are to instead be +1 that value, so bits won't get reused.  A later
commit will renumber the bits in op.h and regexp.h, but for now things
are left as-is there, which means the base variables in those two files
must subtract one to compensate for the +1