[LV] Fixing versioning-for-unit-stide of loops with small trip count

This patch fixes D81345 and PR46652.

If a loop with a small trip count is compiled w/o -Os/-Oz, Loop Access Analysis
still generates runtime checks for unit strides that will version the loop.

In such cases, the loop vectorizer should either re-run the analysis or bail-out
from vectorizing the loop, as done prior to D81345. The latter is applied for
now as the former requires refactoring.

Differential Revision: https://reviews.llvm.org/D83470