Add tie-breaker for register class sorting in getSuperRegForSubReg

llvm::stable_sort is apparently not sufficient.

Use the same tie-breaker/sorting style as TopoOrderRC fix bot failures.

E.g.

http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/19401/steps/test-check-all/logs/stdio

llvm-svn: 370267