[LV] Clamp the VF to the trip count

Summary:
When the MaxVectorSize > ConstantTripCount, we should just clamp the
vectorization factor to be the ConstantTripCount.
This vectorizes loops where the TinyTripCountThreshold >= TripCount < MaxVF.

Earlier we were finding the maximum vector width, which could be greater than
the trip count itself. The Loop vectorizer does all the work for generating a
vectorizable loop, but in the end we would always choose the scalar loop (since
the VF > trip count). This allows us to choose the VF keeping in mind the trip
count if available.

This is a fix on top of rL312472.

Reviewers: Ayal, zvi, hfinkel, dneilson

Reviewed by: Ayal

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37702

llvm-svn: 313046