[tilegx] For TwoByte operation, the length should be multiplied by two.
[tilegx] Fix LCodeGen::PrepareKeyedOperand bug.

  LCodeGen::PrepareKeyedOperand was wrong when handling Smi (v8 boxed
  number format).

    Also fixed MacroAssembler::target_address_at bug. The decoded bits
  needs sign extension. Although our vaddress is always <=42bit, but for v8,
  there are some bogus address, for example 0xfffffffffffe0000 generated by
  ExternalReference::new_space_mask.