Get the ClassLoader from the DexCache in ReferenceTypePropagation

We were passing it down as a parameter when we can simply get it
from the dex cache. Passing it down as a parameter brings the
possibility of using the wrong class loader and can be the source
of bugs.

Test: art/test/testrunner/testrunner.py --host --64 --optimizing -b
Change-Id: I7fbf3d6f79186d4eb695c77b876291d02e3170ba