BBVectorize: Don't store candidate pairs in a std::multimap

This is another commit on the road to removing std::multimap from
BBVectorize. This gives an ~1% speedup on the csa.ll test case
in PR15222.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175215 91177308-0d34-0410-b5e6-96231b3b80d8