[SystemZ] Temporarily disable high VFs with integer div/rem.

Until mischeduler is clever enough to avoid spilling in a vectorized loop
with many (scalar) DLRs it is better to avoid high vectorization factors (8
and above).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344129 91177308-0d34-0410-b5e6-96231b3b80d8