ScheduleOpt: Do not crash on statements with empty iteration domains

Statements with an empty iteration domain may not have a schedule assigned by
the isl schedule optimizer. As Polly expects each statement to have a schedule,
we keep the old schedule for such statements.

This fixes http://llvm.org/PR15645`

Reported-by: Johannes Doerfert  <johannesdoerfert@gmx.de>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@179233 91177308-0d34-0410-b5e6-96231b3b80d8