Update vcompress.vm to not write vstart with 0 upon completion

Vmcompress.vm requires vstart==0, so writing vstart with 0 is redundant.

To do this, spin off VI_LOOP_END_BASE from VI_LOOP_END. VI_LOOP_END
will contain VI_LOOP_END_BASE as well as a write of 0 to vstart.

See #1623 for full discussion.