Switch a std::map to a DenseMap in CodeGenRegisters.

Switch a std::map to a DenseMap in CodeGenRegisters.

The keys of the map are unique by pointer address, so there's no need
to use the llvm::less comparator. This allows us to use DenseMap
instead, which reduces tblgen time by 20% on my stress test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230769 91177308-0d34-0410-b5e6-96231b3b80d8