GH-136410: Faster side exits by using a cold exit stub (GH-136411)

Python/jit.c

@@ -546,7 +546,7 @@ _PyJIT_Compile(_PyExecutorObject *executor, const _PyUOpInstruction trace[], siz
     group->emit(code, data, executor, NULL, &state);
     code += group->code_size;
     data += group->data_size;
-    assert(trace[0].opcode == _START_EXECUTOR);
+    assert(trace[0].opcode == _START_EXECUTOR || trace[0].opcode == _COLD_EXIT);
     for (size_t i = 0; i < length; i++) {
         const _PyUOpInstruction *instruction = &trace[i];
         group = &stencil_groups[instruction->opcode];