[LoopUnswitch] Remove BFI usage (dead code)

Chandler mentioned at the last social that the need for BFI in the new pass manager was causing a slight hiccup for this pass.  Given this code has been checked in, but off for over a year, it makes sense to just remove it for now.

Note that there's nothing wrong with the general idea - it's actually a quite good one - and once we have the infrastructure in place to implement this without the full recompuation on every loop, we absolutely should.

llvm-svn: 294715