Verify profile data confirms large loop trip counts.

Summary:
Loops with inequality comparers, such as:

   // unsigned bound
   for (unsigned i = 1; i < bound; ++i) {...}

have getSmallConstantMaxTripCount report a large maximum static
trip count - in this case, 0xffff fffe. However, profiling info
may show that the trip count is much smaller, and thus
counter-recommend vectorization.

This change:
- flips loop-vectorize-with-block-frequency on by default.
- validates profiled loop frequency data supports vectorization,
  when static info appears to not counter-recommend it. Absence
  of profile data means we rely on static data, just as we've
  done so far.

Reviewers: twoh, mkuper, davidxl, tejohnson, Ayal

Reviewed By: davidxl

Subscribers: bkramer, llvm-commits

Differential Revision: https://reviews.llvm.org/D42946

llvm-svn: 324543