Pointer fixes/improvements.

Allow <, <=, >, >= comparisons of pointers
Allow explicit type-casting of pointers to and from integers
Fix bug in handling expressions of the form "int + ptr" ("ptr + int"
  was fine).
Fix a bug in TypeCastExpr where varying -> uniform typecasts
  would be allowed (leading to a crash later)

expr.h

@@ -502,6 +502,7 @@ public:
     Expr *TypeCheck();
     Expr *Optimize();
     int EstimateCost() const;
+    Symbol *GetBaseSymbol() const;
     llvm::Constant *GetConstant(const Type *type) const;
 
     const Type *type;