A concrete case for potential ref-pruning chances

[dlee992]

code of typed.List's get_item:

@overload(operator.getitem)
def impl_getitem(l, index):
    ...
    if index in index_types:
        if IS_NOT_NONE:
            def integer_non_none_impl(l, index):
                castedindex = _cast(index, indexty)
                handledindex = handle_index(l, castedindex)
                status, item = _list_getitem(l, handledindex)
                if status == ListStatus.LIST_OK:
                    return _nonoptional(item)
                else:
                    raise AssertionError("internal list error during getitem")
            return integer_non_none_impl

IR of integer_non_none_impl:

entry:
  tail call void @NRT_incref(i8* %arg.l.0)
  tail call void @NRT_incref(i8* %arg.l.0), !noalias !22
  ...
  br i1 %.27.i, label %B0.if, label %B20.endif.i

B20.endif.i:                                      ; preds = %entry
  ...
  tail call void @NRT_decref(i8* %arg.l.0), !noalias !22
  ...
  br i1 %.60.not.i, label %B46.endif, label %B0.if

common.ret:                                       ; preds = %B46.endif, %B0.if
  %common.ret.op = phi i32 [ 0, %B46.endif ], [ 1, %B0.if ]
  ret i32 %common.ret.op

B0.if:                                            ; preds = %B20.endif.i, %entry
  %l.sroa.0.0.i = phi i8* [ %arg.l.0, %entry ], [ null, %B20.endif.i ]
  tail call void @NRT_decref(i8* %l.sroa.0.0.i), !noalias !22
  store ..., ... %excinfo, align 8, !numba_exception_output !13
  br label %common.ret

B46.endif:                                        ; preds = %B20.endif.i
  ...
  tail call void @NRT_decref(i8* %arg.l.0)
  ...
  br label %common.ret

Ideally, we should remove all refcount calls of %arg.l.0 and %l.sroa.0.0.i, since %l.sroa.0.0.i is null or %arg.l.0.
However, current ref-pruning algorithm can't remove any of them.