Improve interpreter performance.

Bug: 8196227

This CL removes the use of DecodedInstruction to read instruction opcode and
operands. It now directly access to each operand according to instruction
format (like VRegA_11x for accessing register vA with format 11x).

It also caches some information used for instrumentation like 'this' object
and current method and add missing check about pending exception.

Change-Id: I8c03c0aea9d75068b89e3cb2c8c12383d7928281