SLPVectorizer: Lazily allocate the map for block numbering.

There is no point in creating it if we're not going to vectorize
anything. Creating the map is expensive as it creates large values.
No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207916 91177308-0d34-0410-b5e6-96231b3b80d8