[LV]Add debug counter support to loop vectorizer

Need to add debug counter support to loop vectorizer to simplify the detection of miscompiled loops
@fhahn

The issue names only the loop vectorizer and does not mention a file, test, or entry point. Locate the loop vectorizer implementation and existing debug-counter conventions first; done means debug counters are supported there and can simplify detecting miscompiled loops.

Written by the indexing model from the issue text.