[VPlan] Remove uneeded needsVectorIV check.

Remove one of the last remaining uses of ::needsVectorIV, preparing for
its removal. Now that usesScalars is available and based on the
information explicit in VPlan, there is no need to use the pre-computed
needsVectorIV.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D123720