BFI: Improve assertion message, since it's actually firing

This assertion is firing because -loop-unroll is failing to preserve
-loop-info (see PR20987).  Improve it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219130 91177308-0d34-0410-b5e6-96231b3b80d8