ART-tests: Remove DX dependency from 626-checker-arm64-scratch-register.

This test contains a single test case, a long method and the CHECK
conditions matching the blocks and registers are dependent on the
exact patterns of the DX generated code. This CL moves the entire
method into smali and enables D8.

Test: art/test.py -r -b --target --64 -t 626-checker-arm64-scratch-register
      Tested locally with --host/--gcstress/--jvm.
Bug: 32545705
Bug: 65168732
Change-Id: If50f38a2af526ffee8ea37a4edb4c8def29a4d84