[LoopFlatten] Bail if we can't perform flattening after IV widening

It can happen that after widening of the IV, flattening may not be possible,
e.g. when it is deemed unprofitable. We were not properly checking this, which
resulted in flattening being applied when it shouldn't, also leading to
incorrect results (miscompilation).

This should fix PR51980 (https://bugs.llvm.org/show_bug.cgi?id=51980)

Differential Revision: https://reviews.llvm.org/D110712