[LV] Fix scalar cost for tail predicated loops

When it comes to the scalar cost of any predicated block, the loop
vectorizer by default regards this predication as a sign that it is
looking at an if-conversion and divides the scalar cost of the block by
2, assuming it would only be executed half the time. This however makes
no sense if the predication has been introduced to tail predicate the
loop.

Original patch by Anna Welker

Differential Revision: https://reviews.llvm.org/D86452