Subzero: Use 'override' as appropriate for C++11.

Not necessary for the LLVM 3.5 merge, but nice to have anyway.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3930
R=jfb@chromium.org, jvoung@chromium.org

Review URL: https://codereview.chromium.org/605123002
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index 4efec93..011b04d 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -470,11 +470,12 @@
   }
   Str << getAsmName() << ":\n";
   for (PhiList::const_iterator I = Phis.begin(), E = Phis.end(); I != E; ++I) {
-    InstPhi *Inst = *I;
-    if (Inst->isDeleted())
+    InstPhi *Phi = *I;
+    if (Phi->isDeleted())
       continue;
     // Emitting a Phi instruction should cause an error.
-    Inst->emit(Func);
+    Inst *Instr = Phi;
+    Instr->emit(Func);
   }
   for (InstList::const_iterator I = Insts.begin(), E = Insts.end(); I != E;
        ++I) {