[LV] Prevent LV to run cost model twice for VF=2

This is a minor fix for LV cost model, where the cost for VF=2 was
computed twice when the vectorization of the loop was forced without
specifying a VF.

Reviewers: xusx595, hsaito, fhahn, mkuper

Reviewed By: hsaito, xusx595

Differential Revision: https://reviews.llvm.org/D48048

llvm-svn: 334840