SLPVectorize: Replace the code that checks for vectorization candidates in successor...

SLPVectorize: Replace the code that checks for vectorization candidates in successor blocks with code that scans PHINodes.
Before we could vectorize PHINodes scanning successors was a good way of finding candidates. Now we can vectorize the phinodes which is simpler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186139 91177308-0d34-0410-b5e6-96231b3b80d8