split branch prediction CPU into its own file

[Jason Lowe-Power]

It would be better to have a separate file for the branch predictor CPU data path than replacing the simple pipelined CPU data path. It's a little unfortunate to have lots of code duplication, but, overall, I think it's better to split this out on its own.

I foresee having many different pipeline designs in the pipelined/ directory:

• cpu-simple.scala: Base pipelined design (e.g., answer to lab3)
• cpu-bp.scala: Pipelined design with a branch predictor
• cpu-seqmem.scala: Pipelined design with sequential instead of asynchronous memory.
• Others????