[Constant Hoisting] Replace the MapVector with a separate Map and Vector to keep...

[Constant Hoisting] Replace the MapVector with a separate Map and Vector to keep track of constant candidates.

This simplifies working with the constant candidates and removes the tight
coupling between the map and the vector.

Related to <rdar://problem/16381500>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204431 91177308-0d34-0410-b5e6-96231b3b80d8