ARM: VIXL32: Implement Invoke, LoadClass, LoadString dispatch.

Implemented dispatch optimizations for InvokeStaticOrDirect,
LoadClass, LoadString (excluding cases that use Literals).
Performed a cleanup of VIXL backend.

Test: export ART_USE_VIXL_ARM_BACKEND=true && \
      mma test-art-host dist && mma test-art-target dist

Change-Id: Ib37a6b7e7657196b13caec999d190be747857c1d