[NFC][LoopVectorize] Make the createStepForVF interface more caller-friendly

The common use case for calling createStepForVF is currently something
like:

  Value *Step = createStepForVF(Builder, ConstantInt::get(Ty, UF), VF);

and it makes more sense to reduce overall lines of code and change the
function to let it create the constant instead. With my patch this
becomes:

  Value *Step = createStepForVF(Builder, Ty, VF, UF);

and the ConstantInt is created instead createStepForVF. A side-effect of
this is that the code in createStepForVF is also becomes simpler.

As part of this patch I've also replaced some calls to getRuntimeVF
with calls to createStepForVF, i.e.

  getRuntimeVF(Builder, Count->getType(), VFactor * UFactor) ->
  createStepForVF(Builder, Count->getType(), VFactor, UFactor)

because this feels semantically better.

Differential Revision: https://reviews.llvm.org/D113122