SLPVectorizer: support slp-vectorization of PHINodes between basic blocks

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184888 91177308-0d34-0410-b5e6-96231b3b80d8
Merge isReachable into isBackedge.

Prefer using RPO.lookup() instead of RPO[] which can mutate the map.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184891 91177308-0d34-0410-b5e6-96231b3b80d8
s/C++0x/C++11/

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184892 91177308-0d34-0410-b5e6-96231b3b80d8