[LoopUnroll] Set correct thresholds for new recently enabled unrolling heuristic.

In r270478, where I enabled the new heuristic I posted testing results,
which I got when explicitly passed the thresholds values via CL options.
However, setting the CL options init-values is not enough to change the
default values of thresholds, so I'm changing them in another place now.

llvm-svn: 271615