[RuntimeLoopUnroller] Add assert that we dont unroll non-rotated loops

Summary:
Cloning basic blocks in the loop for runtime loop unroller depends on loop being
in rotated form (i.e. loop latch target is the exit block).
Assert that this is true, so that callers of runtime loop unroller pass in
canonical loops.
The single caller of this function has that check recently added:
https://reviews.llvm.org/rL301239

Reviewers: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32801

llvm-svn: 302058