[LV] Move runtime pointer size check to LVP::plan().

This removes the need for the remaining doesNotMeet check and instead
directly checks if there are too many runtime checks for vectorization
in the planner.

A subsequent patch will adjust the logic used to decide whether to
vectorize with runtime to consider their cost more accurately.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D98634