Make the split function use StringRef::split.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208723 91177308-0d34-0410-b5e6-96231b3b80d8
Save the optimization level the subtarget was created with in a
member variable and sink the initialization of crbits into the
subtarget feature reset code.

No functional change, but this refactor will be used in a future
commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208726 91177308-0d34-0410-b5e6-96231b3b80d8