Clean up class resolution and lookup.

Simplify the code and avoid read barriers when appropriate.
Relax class status check in ArtField::GetDeclaringClass()
because we can see ClassStatus::kIdx from the from-space
class object for kReadBarrierOption=kWithoutReadBarrier.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I83886a64fe5a99a1c3c30eab3b35dae449e6b4bc