Improve internal API for fetching original instruction, replacing `_Py_GetBaseOpcode`

[markshannon]

The function _Py_GetBaseOpcode returns the original opcode for an instruction, even if that instruction has been instrumented.
However it doesn't work for ENTER_EXECUTOR and doesn't allow for the op.arg to have been changed.

We should add a new _Py_GetBaseInstruction which handles ENTER_EXECUTOR as well, and is efficient enough to use in the few places where it is used outside of an assert, such as in is_resume which is used in gen_close.

The new API will be either

_Py_CODEUNIT _Py_GetBaseInstruction(PyCodeObject *code, int i)

or

_Py_CODEUNIT _Py_GetBaseInstruction(PyCodeObject *code, _Py_CODEUNIT *)

depending on which is the most convenient for the current use cases.

Linked PRs

• GH-122390: Replace _Py_GetbaseOpcode with _Py_GetBaseCodeUnit #122942