Refactor OSR related code to prepare for "true" OSR.

- Make the compiler restore all callee-save registers.
- Make the compiler return any value in a core register: this simplifies
  the current stub, and will also avoid having to look at the return
  type (and reading the shorty) when returning to an nterp frame.
- Add OsrData and offsets of its members to be used by nterp.

Test: test.py
Bug: 27094810
Change-Id: Ifa4f4877ab8b1f0c6a96feccea30c909942eb2fa
13 files changed