gh-116968: Reimplement Tier 2 counters (#117144)

Introduce a unified 16-bit backoff counter type (``_Py_BackoffCounter``),
shared between the Tier 1 adaptive specializer and the Tier 2 optimizer. The
API used for adaptive specialization counters is changed but the behavior is
(supposed to be) identical.

The behavior of the Tier 2 counters is changed:
- There are no longer dynamic thresholds (we never varied these).
- All counters now use the same exponential backoff.
- The counter for ``JUMP_BACKWARD`` starts counting down from 16.
- The ``temperature`` in side exits starts counting down from 64.

Python/ceval.c

@@ -4,6 +4,7 @@
 
 #include "Python.h"
 #include "pycore_abstract.h"      // _PyIndex_Check()
+#include "pycore_backoff.h"
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
 #include "pycore_cell.h"          // PyCell_GetRef()
 #include "pycore_ceval.h"
@@ -822,7 +823,7 @@ resume_frame:
             _PyBinaryOpCache *cache = (_PyBinaryOpCache *)(next_instr+1);
             /* Prevent the underlying instruction from specializing
              * and overwriting the instrumentation. */
-            INCREMENT_ADAPTIVE_COUNTER(cache->counter);
+            PAUSE_ADAPTIVE_COUNTER(cache->counter);
         }
         opcode = original_opcode;
         DISPATCH_GOTO();
@@ -1099,7 +1100,7 @@ exit_to_trace:
         printf("SIDE EXIT: [UOp ");
         _PyUOpPrint(&next_uop[-1]);
         printf(", exit %u, temp %d, target %d -> %s]\n",
-               exit_index, exit->temperature, exit->target,
+               exit_index, exit->temperature.as_counter, exit->target,
                _PyOpcode_OpName[_PyCode_CODE(_PyFrame_GetCode(frame))[exit->target].op.code]);
     }
 #endif