Add support for passing and returning vectors in accordance with the x86 calling convention.

src/IceCfg.h

Index: src/IceCfg.h
diff --git a/src/IceCfg.h b/src/IceCfg.h
index dbf08c66ba3e5841223a47482c653a84914f871e..856b145c3ebfffd94ec7858ce73547092809c168 100644
--- a/src/IceCfg.h
+++ b/src/IceCfg.h
@@ -69,6 +69,7 @@ public:
   // Manage arguments to the function.
   void addArg(Variable *Arg);
   const VarList &getArgs() const { return Args; }
+  VarList &getArgs() { return Args; }
 
   // Miscellaneous accessors.
   TargetLowering *getTarget() const { return Target.get(); }
@@ -86,6 +87,7 @@ public:
   void placePhiStores();
   void deletePhis();
   void doAddressOpt();
+  void doArgLowering();
   void genCode();
   void genFrame();
   void livenessLightweight();