[SystemZ] Improve handling and cost estimates of vector integer div/rem

Enable the DAG optimization that converts vector div/rem with constants into
multiply+shifts sequences by expanding them early. This is needed since
ISD::SMUL_LOHI is 'Custom' lowered on SystemZ, and will therefore not be
available to BuildSDIV after legalization.

Better cost values for these instructions based on how they will be
implemented (a constant divisor is cheaper).

Review: Ulrich Weigand
https://reviews.llvm.org/D53196

llvm-svn: 345321