[LoopFlatten] Updating Phi nodes after IV widening

In rG6a076fa9, a problem with updating the old/narrow phi nodes after IV
widening was introduced. If after widening of the IV the transformation is
*not* applied, the narrow phi node was incorrectly modified, which should only
happen if flattening happens. This can be seen in the added test widen-iv2.ll,
which incorrectly had 1 incoming value, but should have its original 2 incoming
values, which is now restored.

Differential Revision: https://reviews.llvm.org/D110234