[LV] Use getMaxRuntimeElementCount for MaxPowerOf2RuntimeVF. (#221219)
Use new getMaxRuntimeElementCount to compute MaxPowerOf2RuntimeVF to
avoid overflow by performing computations in uint64_t.
This fixes an overflow in the test added in #219290. We now correctly
determine that a tail is needed for a low trip count loop.
PR: https://github.com/llvm/llvm-project/pull/221219