regexp.h: Move regex flag bit positions.

regexp.h: Move regex flag bit positions.

This moves three bits to create a block of unused bits at the beginning.
The first bit had to be moved to make space for other uses that are
coming in future commits.  This breaks binary compatibility, so might as
well move the other two bits so that all the unused bits are
consolidated at the beginning.

This pool of unused bits is the boundary between the bits that are
common to op.h and regexp.h (and in op_reg_common.h) and those that are
separate.  It's best to have all the unused bits there, so when we need
to use one, it can be taken from either side, as needed, without us
being trapped into having an available bit, but of the wrong kind.