TableGen: Switch from a std::map to a DenseMap in CodeGenSubRegIndex. NFC

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

utils/TableGen/CodeGenRegisters.h

@@ -74,8 +74,7 @@ namespace llvm {
     std::string getQualifiedName() const;
 
     // Map of composite subreg indices.
-    typedef std::map<CodeGenSubRegIndex *, CodeGenSubRegIndex *,
-                     deref<llvm::less>> CompMap;
+    typedef DenseMap<CodeGenSubRegIndex *, CodeGenSubRegIndex *> CompMap;
 
     // Returns the subreg index that results from composing this with Idx.
     // Returns NULL if this and Idx don't compose.