[LoopFlatten] Widen the IV

Widen the IV to the widest available and legal integer type, which makes this
transformations always safe so that we can skip overflow checks.

Motivation is to let this pass trigger on 64-bit targets too, and this is the
last patch in a serie to achieve this: D90402 moves pass LoopFlatten to just
before IndVarSimplify so that IVs are not already widened, D90421 factors out
widening from IndVarSimplify into Utils/SimplifyIndVar so that we can also use
it in LoopFlatten.

Differential Revision: https://reviews.llvm.org/D90640