ARM64: Improve Mterp.

Several straight-forward optimizations:
  - use ubfx instead of SHR+AND,
  - do not mask shifting distance,
  - use 64-bit LDRSH to avoid subsequent sign extension,
  - use CBNZ instead of CMP+BNE for null checks,
  - style: use SXTW alias instead of explicit SBFM.

Test: Run ART test suite on Nexus 9 with the interpreter.
Change-Id: Ibcc11ce7f455432ecb789f727da21f269f8370f0