Change check cast entrypoint to check instance of

Reduces code size since we do not need to reload class before
calling slow path.

TODO: Delete read barriers in the check cast code since the slow
path will retry with the proper read barriers if the check fails.

Bug: 12687968
Bug: 29516974

Test: test-art-host + test-art-target with CC

Change-Id: Ia4eb9bbe3fe2d2016e44523cf0451210828d7b88