Try to squeeze some delay out of the critical path and clean up operand fetch

- Remove logic from operand fetch that checked for PC sources and move it
into decode stage.  This eliminates a priority encoded in favor of a
multiplexer (both source 1 & 2)
- Compute adjusted PC a cycle earlier and latch through
- Remove extraneous (default) inputs from multiplexers
- Also cleaned up some global copy/replace typos