Switch-interpreter: Add helpers to get opcode arguments.

Add trivial helpers to get opcodes arguments (A/B/C).

Ensure the generic getters in Instruction can take
advantage of knowing the format at compile-time.

As the first step, I have simplified the code only where
the argument is used as index for SetVReg/GetVReg calls.

Test: ART_USE_CXX_INTERPRETER=true ./art/test.py -b --host
Test: ART_USE_CXX_INTERPRETER=true ./art/tools/run-jdwp-tests.sh \
      --mode=host --variant=X32 --debug --no-jit
Change-Id: I765c0eb16438806c3624fabe3aade040c7bb3387
5 files changed