Use HeapReference for SafeGetClass

Make sure to use heap refernece here so the value read isn't bogus
for heap poisoning. Fixes heap poisoning tests that were broken for
implicit null checks.

Bug: 31023171
Test: test/testrunner/testrunner.py --interpreter --optimizing -j20 --host --verbose

Change-Id: Ia67cdb304be205773d781251235f8b0b4bf7c0bd