prevent array subscript expression if base/index is not rewritable

The following command failed:
  trace.py -U 'r::_do_fork (retval == -11) "%llu", ((struct task_struct *)bpf_get_current_task())->signal->rlim[RLIMIT_NPROC].rlim_cur'

as rewriter generates code like
  __data.v0 = (unsigned long long)((struct task_struct *)bpf_get_current_task())->signal->rlim[RLIMIT_NPROC))); _val; }).rlim_cur;

Let us prevent rewriting if either base or index is not rewritable and this fixed the issue.

Signed-off-by: Yonghong Song <yhs@fb.com>