BBVectorize: Fix anomalous capital letter in comment

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173579 91177308-0d34-0410-b5e6-96231b3b80d8
BBVectorize: Add a additional comment about the cost computation

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173580 91177308-0d34-0410-b5e6-96231b3b80d8