[LoopVectorizer] Improve handling of branches during cost estimation.

The cost for a branch after vectorization is very different depending on if
the vectorizer will if-convert the block (branch is eliminated), or if
scalarized and predicated blocks will be produced (branch duplicated before
each block). There is also the case of remaining scalar branches, such as the
back-edge branch.

This patch handles these cases differently with TTI based cost estimates.

Review: Matthew Simpson
https://reviews.llvm.org/D31175

llvm-svn: 300058