[LoopFlatten] Simplify loops so that the pass can operate on unsimplified loops.

The loop flattening pass requires loops to be in simplified form. If the
loops are not in simplified form, the pass cannot operate. This patch
simplifies all loops before flattening. As a result, all loops will be
simplified regardless of whether anything ends up being flattened.

This change was inspired by observing a certain loop that was not flatten
because the loops were not in simplified form. This loop is added as a
test to verify that it is now flattened.

Differential Revision: https://reviews.llvm.org/D102249

Change-Id: I45bcabe70fb99b0d89f0effafc82eb9e0585ec30