Don't block quick callee saved registers for optimizing.

This change builds on:
https://android-review.googlesource.com/#/c/118983/

- Also fix x86_64 assembler bug triggered by this change.
- Fix (and improve) x86's backend byte register usage.
- Fix a bug in baseline register allocator: a fixed
  out register must prevent inputs from allocating it.

Change-Id: I4883862e29b4e4b6470f1823cf7eab7e7863d8ad