Recommit "[VPlan] Introduce recipe to build scalar steps."

This reverts the revert commit ff93260b.

The underlying issue causing the PPC bot failures has been fixed in
cbaac147 and a corresponding test case has been added in
ad2cad1c.

Original message:

    This patch adds a new VPScalarIVStepsRecipe to handle building scalar
    steps.

    In the first patch, it only handles the case where there is no vector
    induction variable needed.

    Reviewed By: Ayal

    Differential Revision: https://reviews.llvm.org/D115953