Simplify SelectRoot's interface, and factor out some common code
from all targets.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55124 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/Alpha/AlphaISelDAGToDAG.cpp b/lib/Target/Alpha/AlphaISelDAGToDAG.cpp
index 89dee2a..0ff735d 100644
--- a/lib/Target/Alpha/AlphaISelDAGToDAG.cpp
+++ b/lib/Target/Alpha/AlphaISelDAGToDAG.cpp
@@ -236,7 +236,7 @@
   DEBUG(BB->dump());
   
   // Select target instructions for the DAG.
-  DAG.setRoot(SelectRoot(DAG.getRoot()));
+  SelectRoot();
   DAG.RemoveDeadNodes();
 }