Fixed issue with aliasing local variables

ISPC++ now produces valid code, or an appropriate error message, for all
of my test cases.

expr.h

@@ -761,6 +761,7 @@ public:
     Symbol *GetBaseSymbol() const;
     Expr *TypeCheck();
     Expr *Optimize();
+    Expr *ReplacePolyType(const PolyType *from, const Type *to);
     void Print() const;
     int EstimateCost() const;
     llvm::Constant *GetConstant(const Type *type) const;