BBVectorize: Eliminate one more restricted linear search

This eliminates one more linear search over a range of
std::multimap entries. This gives a 22% speedup on the
csa.ll test case from PR15222.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174893 91177308-0d34-0410-b5e6-96231b3b80d8