[RewriteStatepointsForGC] Replace std::map with DenseMap

I'd done the work of extracting the typedef in a previous commit, but didn't actually change it.  Hopefully this will make any subtle changes easier to isolate.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230081 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed