Multi-cycle pipeline fixes

- Don't have fields in decoded instruction depend combinatorially on other
fields in the same structure. Verilator complains this is a loop (it
technically isn't, but this is cleaner)
- mx2_instruction was latched incorrectly
- Make sure multi-cycle pipeline only latched instructions that are bound
for it (otherwise, other instructions were slipping in)
- Writeback allocate logic was broken.  Shifter was going wrong direction
and there was an off-by-one in the multi-cycle pipeline latency.