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.
[Fix] Dead statements should not confuse the RTC generation

  This fixes http://llvm.org/bugs/show_bug.cgi?id=21166 .

Differential Revision: http://reviews.llvm.org/D5623