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>
llvm-svn: 179233