as we don't necessarily need to do this yet - though we could move
the base class to the TargetMachine as it isn't subtarget dependent.
This reverts commit r232103.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232665 91177308-0d34-0410-b5e6-96231b3b80d8