SKW: Static branch prediction.

[ben-marshall]

Currently, all control flow changes have a 5 cycle penalty, since they wait until the instruction reaches writeback before triggering the change.
Two improvements are possible within the scope of a microcontroller like the SCARV-CPU:

• Decode stage taking of jump with immediate instructions.
• "Always taken" or "predict taken if backwards" branch prediction.

Step 1, tracked here, will just implement static early branching in the decode stage for control flow changes where it is possible.

• Instructions eligible: c.j,c.jal,jal.