frontend: Clean-up instruction frontend

The instuction frontend has become an increasingly messy part an needed
cleaning-up. The current solution contains 2 x 32 bit instruction data
fifos and 1 x 64 bit address fifo. Hence, it should be significantly
more area efficient that the previous one. The interface to `id_stage`
is a ready/valid handshake. The credit based system has been replaced in
favour of a replay mechanism as it was very brittle and overly
pessimistic.

Branch-prediction has been cleaned up: The front-end was also partially
predicting on jumps, this could have potentially let to performance bugs
if the branch detection wasn't correct in the frontend.