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

It turns out we iterate over this map a fair amount and the order
matters for clang to be deterministic. See:

  http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160919/391315.html

This reverts r279875.

llvm-svn: 282040

llvm/utils/TableGen/CodeGenRegisters.h

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