[VPlan] Reject loops without computable backedge taken counts

getOrCreateTripCount is used to generate code for the outer loop, but it
requires a computable backedge taken counts. Check that in the VPlan
native path.

Reviewers: Ayal, gilr, rengolin, sguggill

Reviewed By: sguggill

Differential Revision: https://reviews.llvm.org/D81088