BBVectorize: Remove the remaining instances of std::multimap

All instances of std::multimap have now been replaced by
DenseMap<K, std::vector<V> >, and this yields a speedup of 5% on the
csa.ll test case from PR15222.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175216 91177308-0d34-0410-b5e6-96231b3b80d8