[rtl] Use IF skid buffer to reduce mis-predict penalty for branch prediction

Nobody has claimed this yet.

When a branch is predicted taken Ibex may have already prefetched the instruction that follows the branch but will throw it away to follow the branch prediction. If the branch was mis-predicted it then needs to refetch the instruction following the branch causing a stall cycle.

The skid buffer in the IF could be used to hold any prefetched instruction that follows a branch and remove the mis-predict penalty in some cases.

Start by locating Ibex's instruction-fetch skid buffer and branch-prediction or misprediction handling in the RTL. Trace how a prefetched instruction after a branch is discarded, then determine whether the buffer can retain it; done means applicable mispredictions no longer incur the refetch stall without breaking existing behavior.

Written by the indexing model from the issue text.