Clean up constructor logic and member access for LoopVectorizeHints.

There are public functions that mutate various members as well as
another private member already, so make all the members private to
avoid the discontinuity and add accessors for the values. Should
be no functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207868 91177308-0d34-0410-b5e6-96231b3b80d8