[VPlan] Check VPValue step in isCanonical (NFCI).

Update the isCanonical() implementations to check the VPValue step
operand instead of the step in the induction descriptor.

At the moment this is NFC, but it enables further optimizations if the
step is replaced by a constant in D147783.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D147891