Implement checkcast for optimizing.

- Ended up not using HTypeCheck because of how
  instanceof and checkcast end up having different logic
  for code generation.

- Fix a x86_64 assembler bug triggered by now enabling
  more methods to be compiled. Difficult to test today
  without b/18117217.

Change-Id: I3022e7ae03befb1d10bea9637ad21fadc430abe0