Check that classes are loaded before updating entrypoints.

We were trying to update the entrypoints of methods taken from classes
that were not yet loaded. This means we were racing the class-loading
and were sometimes reading methods that had not been fully initialized
yet.

Test: stress --cpu 60
Test: while ./test/run-test --host --jit 1916; do ; done
Bug: 66933582
Change-Id: Ib2aea9c9865f838af525366efa88adecc1744865