gh-157740: Fix crash when multiple threads execute the same code with the JIT enabled

[cocolato]

When threads share a code object, one thread can install an executor after the tracer records an instruction's argument but before it translates the instruction. The opcode is then ENTER_EXECUTOR, but the translator uses the recorded oparg as the executor index, which can read past the executor array and crash.

Reading the executor index from this_instr->op.arg when handling ENTER_EXECUTOR fixes this.

• Issue: AddressSanitizer: heap-buffer-overflow Python/optimizer.c:709:39 in _PyJit_translate_single_bytecode_to_trace #157740