SLPVectorization:  Add a basic support for cross-basic block slp vectorization.

We collect gather sequences when we vectorize basic blocks. Gather sequences are excellent
hints for vectorization of other basic blocks.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184444 91177308-0d34-0410-b5e6-96231b3b80d8