Support GVN for HLoadClass::LoadKind::kJitTableAddress.

Fixes performance regressions seen in eg Dhrystone.

Also add comment on why a class may not be found when sharpening.

Test: manual Dhrystone run, performance recovers
Test: ART_TEST_JIT=true test-art-host-run-test-jit
Change-Id: I8e879f1c390f83e8bc930f343beb7b4a41c2f190