O3: Keep around the last committed instruction and use for squashing.

Before this, 0 was used for the youngest sequence number to squash
and there was a race to rename. In the case of a instruction marked
serializeAfter that squashes the ROB is empty and 0 is passed to the rename.
If multiple instructions commited that same cycle rename it could undo some
renames beacuse of the 0 being passed which would undo some state.