Instead of passing in an unsigned value for the optimization level, use an enum,

Instead of passing in an unsigned value for the optimization level, use an enum,
which better identifies what the optimization is doing. And is more flexible for
future uses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70440 91177308-0d34-0410-b5e6-96231b3b80d8