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/X86/X86ISelDAGToDAG.cpp b/lib/Target/X86/X86ISelDAGToDAG.cpp
index 13ed07c..21837d5 100644
--- a/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -592,7 +592,7 @@
   DOUT << "===== Instruction selection begins:\n";
   Indent = 0;
 #endif
-  DAG.setRoot(SelectRoot(DAG.getRoot()));
+  SelectRoot();
 #ifndef NDEBUG
   DOUT << "===== Instruction selection ends:\n";
 #endif