SLP: Document the scalarization cost method.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179479 91177308-0d34-0410-b5e6-96231b3b80d8
Document the SLP infrastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179480 91177308-0d34-0410-b5e6-96231b3b80d8