Subzero: Convert a few getName() methods to return a const string ref.

src/IceOperand.h

Index: src/IceOperand.h
diff --git a/src/IceOperand.h b/src/IceOperand.h
index 13ae14c4ff3664f5cb9dbb58fe132146d03d8c29..1f2609fbe7fdcd44777755343e445c055407f570 100644
--- a/src/IceOperand.h
+++ b/src/IceOperand.h
@@ -223,7 +223,7 @@ public:
   }
 
   RelocOffsetT getOffset() const { return Offset; }
-  IceString getName() const { return Name; }
+  const IceString &getName() const { return Name; }
   void setSuppressMangling(bool Value) { SuppressMangling = Value; }
   bool getSuppressMangling() const { return SuppressMangling; }
   using Constant::emit;