[LoopFlatten] Widen IV, support ZExt.

I disabled the widening in fa5cb4b9 because it run in an assert, which was
related to replacing values with different types. I forgot that an extend could
also be a zero-extend, which I have added now. This means that the approach now
is to create and insert a trunc value of the outerloop for each user, and use
that to replace IV values.

Differential Revision: https://reviews.llvm.org/D91690