Subzero: Fix some issues related to legalization and undef handling.

src/IceOperand.h

Index: src/IceOperand.h
diff --git a/src/IceOperand.h b/src/IceOperand.h
index bbd6688fe5cd684c436c5bf1abc31eda095b5ae3..73e51e5138387d8c1cbcf76159db5d5bf75c894a 100644
--- a/src/IceOperand.h
+++ b/src/IceOperand.h
@@ -217,10 +217,8 @@ public:
   }
 
   using Constant::emit;
-  virtual void emit(GlobalContext *Ctx) const {
-    Ostream &Str = Ctx->getStrEmit();
-    Str << "undef";
-  }
+  // The target needs to implement this.
+  virtual void emit(GlobalContext *Ctx) const;
 
   using Constant::dump;
   virtual void dump(GlobalContext *Ctx) const {