[IRCE] Do not process empty safe ranges

IRCE should not apply when the safe iteration range is proved to be empty.
In this case we do unneeded job creating pre/post loops and then never
go to the main loop.

This patch makes IRCE not apply to empty safe ranges, adds test for this
situation and also modifies one of existing tests where it used to happen
slightly.

Reviewed By: anna
Differential Revision: https://reviews.llvm.org/D38577

llvm-svn: 315437