[SystemZ] Implementation of getUnrollingPreferences().

This commit enables more unrolling for SystemZ by implementing the
SystemZTargetTransformInfo::getUnrollingPreferences() method.

It has been found that it is better to only unroll moderately, so the
DefaultUnrollRuntimeCount has been moved into UnrollingPreferences in order
to set this to a lower value for SystemZ (4).

Reviewers: Evgeny Stupachenko, Ulrich Weigand.
https://reviews.llvm.org/D24451

llvm-svn: 282570