[LV] Compute register usage for interleaving on VPlan.
Add a version of calculateRegisterUsage that works estimates register
usage for a VPlan. This mostly just ports the existing code, with some
updates to figure out what recipes will generate vectors vs scalars.
There are number of changes in the computed register usages, but they
should be more accurate w.r.t. to the generated vector code.
There are the following changes:
* Scalar usage increases in most cases by 1, as we always create a
scalar canonical IV, which is alive across the loop and is not
considered by the legacy implementation
* Output is ordered by insertion, now scalar registers are added first
due the canonical IV phi.
* Using the VPlan, we now also more precisely know if an induction will
be vectorized or scalarized.
