[LV] Create & use VPScalarIVSteps for all scalar users.

This patch is a follow-up to D115953. It updates optimizeInductions
to also introduce new VPScalarIVStepsRecipes if an IV has both vector
and scalar uses.

It updates all uses that only need scalar values to use the newly
created recipe for the scalar steps.

This completes untangling of VPWidenIntOrFpInductionRecipe
code-generation. Now the recipe *only* creates the widened vector
values, as it says on the tin.

The code to genereate IR has been moved directly to
VPWidenIntOrFpInductionRecipe::execute.

Note that the recipe has been updated to hold a reference to
ScalarEvolution, which is needed to expand the step, until we can place
the corresponding SCEV expansion in the pre-header.

Depends on D120827.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D120828