[MISched] Require asserts and AArch64 registered target for test.

Fixes failure at https://lab.llvm.org/buildbot/#/builders/124/builds/7472:

```
llc: Unknown command line argument '-debug-only=machine-scheduler'. Try: '/home/buildbot/as-worker-91/clang-with-lto-ubuntu/build/stage1/bin/llc --help'
```

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D152703