Add a runtime unrolling parameter to the LoopUnroll pass constructor

As with the other loop unrolling parameters (the unrolling threshold, partial
unrolling, etc.) runtime unrolling can now also be controlled via the
constructor. This will be necessary for moving non-trivial unrolling late in
the pass manager (after loop vectorization).

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194027 91177308-0d34-0410-b5e6-96231b3b80d8