Major refactoring of the branch predictor unit.
- Clearer control flow of the main branch predictor
- Remove `uncondBranch` and `btbUpdate` functions in favour of a
common `historyUpdate` function. There is now only one lookup
function for conditional branches and the new `historyUpdate` for
speculative history update.
- Added a new *target provider* class.
- More expressive statistics depending on the different branch types.
- Cleanup the branch history management
Change-Id: I21fa555b5663e4abad7c836fc1d41a9c8b205263
Signed-off-by: David Schall <david.schall@ed.ac.uk>
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