BBVectorize: Make the bookkeeping to support full cycle checking less expensive

BBVectorize: Make the bookkeeping to support full cycle checking less expensive

By itself, this does not have much of an effect, but only because in the default
configuration the full cycle checks are used only for small problem sizes.
This is part of a general cleanup of uses of iteration over std::multimap
ranges only for the purpose of checking membership.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174856 91177308-0d34-0410-b5e6-96231b3b80d8