cpu: Restructure BTB

- A new abstract BTB class is created to enable different BTB
  implementations. The new BTB class gets its own parameter
  and stats.
- An enum is added to differentiate branch instruction types.
  This enum is used to enhance statistics and BPU management.
- The existing BTB is moved into `simple_btb` as default.
- An additional function is added to store the static instruction in
  the BTB. This function is used for the decoupled front-end.
- Update configs to match new BTB parameters.

Change-Id: I99b29a19a1b57e59ea2b188ed7d62a8b79426529
Signed-off-by: David Schall <david.schall@ed.ac.uk>

configs/common/cores/arm/HPI.py

@@ -1679,7 +1679,13 @@ class HPI_MMU(ArmMMU):
     dtb = ArmTLB(entry_type="data", size=256)
 
 
+class HPI_BTB(SimpleBTB):
+    numEntries = 128
+    tagBits = 18
+
+
 class HPI_BP(TournamentBP):
+    btb = HPI_BTB()
     localPredictorSize = 64
     localCtrBits = 2
     localHistoryTableSize = 64
@@ -1687,8 +1693,6 @@ class HPI_BP(TournamentBP):
     globalCtrBits = 2
     choicePredictorSize = 1024
     choiceCtrBits = 2
-    BTBEntries = 128
-    BTBTagSize = 18
     RASSize = 8
     instShiftAmt = 2