[LoopFlatten] Fix bug where SCEVCouldNotCompute object is used

The SCEV method getBackedgeTakenCount() returns a SCEVCouldNotCompute
object if the backedge-taken count is unpredictable. This fix ensures
there is no longer an attempt to use such an object to find the trip
count.

Patch by: Rosie Sumpter.

Differential Revision: https://reviews.llvm.org/D106970