[SystemZ] Improve getCurrCycleIdx() in SystemZHazardRecognizer.

getCurrCycleIdx() returns the decoder cycle index which the next candidate SU
will be placed on.

This patch improves this method by passing the candidate SU to it so that if
SU will begin a new group, the index of that group is returned instead.

Review: Ulrich Weigand
llvm-svn: 326880