This mapping is between pointers, which DenseMap is particularly good
at. Most targets aren't really affected, but if there's a lot of
subregister composition this can shave off a good chunk of time from
generating registers.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279875 91177308-0d34-0410-b5e6-96231b3b80d8