Track IR ordering of SelectionDAG nodes 1/4.

Use a field in the SelectionDAGNode object to track its IR ordering.
This adds fields and utility classes without changing existing
interfaces or functionality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182701 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 9799568..32035b0 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -869,7 +869,7 @@
   UnusedArgNodeMap.clear();
   PendingLoads.clear();
   PendingExports.clear();
-  CurDebugLoc = DebugLoc();
+  CurInst = NULL;
   HasTailCall = false;
 }
 
@@ -951,14 +951,14 @@
   if (isa<TerminatorInst>(&I))
     HandlePHINodesInSuccessorBlocks(I.getParent());
 
-  CurDebugLoc = I.getDebugLoc();
+  CurInst = &I;
 
   visit(I.getOpcode(), I);
 
   if (!isa<TerminatorInst>(&I) && !HasTailCall)
     CopyToExportRegsIfNeeded(&I);
 
-  CurDebugLoc = DebugLoc();
+  CurInst = NULL;
 }
 
 void SelectionDAGBuilder::visitPHI(const PHINode &) {