gh-91719: Reload opcode on unknown error so that C can optimize the dispatching in ceval.c (GH-94364) (#94453)

(cherry picked from commit ea39b77)

Co-authored-by: neonene <53406459+neonene@users.noreply.github.com>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2022-06-29-22-18-36.gh-issue-91719.3APYYI.rst

@@ -0,0 +1,2 @@
+Reload ``opcode`` when raising ``unknown opcode error`` in the interpreter main loop,
+for C compilers to generate dispatching code independently.

Python/ceval.c

@@ -5661,6 +5661,9 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
 #else
         EXTRA_CASES  // From opcode.h, a 'case' for each unused opcode
 #endif
+            /* Tell C compilers not to hold the opcode variable in the loop.
+               next_instr points the current instruction without TARGET(). */
+            opcode = _Py_OPCODE(*next_instr);
             fprintf(stderr, "XXX lineno: %d, opcode: %d\n",
                     _PyInterpreterFrame_GetLine(frame),  opcode);
             _PyErr_SetString(tstate, PyExc_SystemError, "unknown opcode");