Optimize suspend checks in optimizing compiler.

- Remove the ones added during graph build (they were added
  for the baseline code generator).
- Emit them at loop back edges after phi moves, so that the test
  can directly jump to the loop header.
- Fix x86 and x86_64 suspend check by using cmpw instead of cmpl.

Change-Id: I6fad5795a55705d86c9e1cb85bf5d63dadfafa2a