gh-115859: Re-enable T2 optimizer pass by default (#116062)

This undoes the *temporary* default disabling of the T2 optimizer pass in gh-115860.

- Add a new test that reproduces Brandt's example from gh-115859; it indeed crashes before gh-116028 with PYTHONUOPSOPTIMIZE=1
- Re-enable the optimizer pass in T2, stop checking PYTHONUOPSOPTIMIZE
- Rename the env var to disable T2 entirely to PYTHON_UOPS_OPTIMIZE (must be explicitly set to 0 to disable)
- Fix skipIf conditions on tests in test_opt.py accordingly
- Export sym_is_bottom() (for debugging)
- Fix various things in the `_BINARY_OP_` specializations in the abstract interpreter:
  - DECREF(temp)
  - out-of-space check after sym_new_const()
  - add sym_matches_type() checks, so even if we somehow reach a binary op with symbolic constants of the wrong type on the stack we won't trigger the type assert

Python/optimizer.c

@@ -1008,8 +1008,8 @@ uop_optimize(
         return err;
     }
     OPT_STAT_INC(traces_created);
-    char *uop_optimize = Py_GETENV("PYTHONUOPSOPTIMIZE");
-    if (uop_optimize == NULL || *uop_optimize > '0') {
+    char *env_var = Py_GETENV("PYTHON_UOPS_OPTIMIZE");
+    if (env_var == NULL || *env_var == '\0' || *env_var > '0') {
         err = _Py_uop_analyze_and_optimize(frame, buffer,
                                            UOP_MAX_TRACE_LENGTH,
                                            curr_stackentries, &dependencies);