Add a fast path to fetch a field in the caller's declaring class.

Call that fast path directly (it's a leaf) and don't update the thread
cache if found. This reduces conflicts in the thread cache.

On arm64, the new fast path shows negligeable regression compared to the
thread local cache.

Test: test.py
Flag: EXEMPT optimization
Change-Id: I67c3873262dfae33f9876eba9907f83f010088e8