LoopVectorizer: Keep track of conditional store basic blocks

Before conditional store vectorization/unrolling we had only one
vectorized/unrolled basic block. After adding support for conditional store
vectorization this will not only be one block but multiple basic blocks. The
last block would have the back-edge. I updated the code to use a vector of basic
blocks instead of a single basic block and fixed the users to use the last entry
in this vector. But, I forgot to add the basic blocks to this vector!

Fixes PR18724.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201028 91177308-0d34-0410-b5e6-96231b3b80d8