[clang][Interp][NFC] Fix Pointer::isZero() for block pointers

We don't need to consider the offset here anymore since we now
have proper integral pointers.

Author: tbaederr

clang/lib/AST/Interp/Pointer.h

@@ -241,13 +241,10 @@ class Pointer {
 
   /// Checks if the pointer is null.
   bool isZero() const {
-    if (Offset != 0)
-      return false;
-
     if (isBlockPointer())
       return asBlockPointer().Pointee == nullptr;
     assert(isIntegralPointer());
-    return asIntPointer().Value == 0;
+    return asIntPointer().Value == 0 && Offset == 0;
   }
   /// Checks if the pointer is live.
   bool isLive() const {