By moving xbm_previous into the SvPVX, we save a U16 from struct xpvbm,

By moving xbm_previous into the SvPVX, we save a U16 from struct xpvbm,
which will likely equate to 4 bytes with padding. (But we add 2 bytes
to the SvPVX buffer...). However, the real win is that xpvbm is now
identical to xpvmg, and all active FBM compiled strings are SvVALID()
true, so now we can eliminate xpvbm, freeing a type for 6 on 5 proxies.

p4raw-id: //depot/perl@29527