Clean up compile time null checks for VarHandle intrinsics.

A few small changes:

- In the instruction simplifier, re-do the `object->IsNullConstant()`
  check as this might have changed after the inliner.

- Make `CanEnsureNotNullAt()` a static member: it is used locally and
  should have local visibility, which was accidentally changed.

Bug: 191765508
Test: art/test.py --host -r
Change-Id: Ib7d88ded4cd73543f66cae75117679a40021c84c