[GMR] Continue my quest to remove linked datastructures from GMR, NFC.

[GMR] Continue my quest to remove linked datastructures from GMR, NFC.

This replaces the next-to-last std::map with a DenseMap. While DenseMap
doesn't yet make tons of sense (there are 32 bytes or so in the value
type), my next change will reduce the value type to a single pointer --
we only need a pointer and 3 bits, and that is exactly what we can have.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242956 91177308-0d34-0410-b5e6-96231b3b80d8