IR: Replace uses of ConstantAggrUniqueMap with ConstantUniqueMap

Now that `ConstantAggrUniqueMap` and `ConstantUniqueMap` work the same
way, change the aggregates to use the new one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215959 91177308-0d34-0410-b5e6-96231b3b80d8