[llvm-mca] Fix wrong call to setCustomStrategy().

Thanks to @waltl for reporting this issue.

I have also added an assert to check for invalid null strategy objects, and I
have reworded a couple of code comments in Scheduler.h


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340545 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed