[ADT] Require base equality in indexed_accessor_iterator::operator==() (#107856)

Similarly to operator<(), equality-comparing iterators from different
ranges must really be forbidden. The preconditions for being able to do
`it1 < it2` and `it1 != it2` (or `it1 == it2` for the matter) ought to
be the same. Thus, there's little sense in keeping explicit base object
comparison in operator==() whilst having this is a precondition in
operator<() and operator-() (e.g. used for std::distance() and such).

Author: andrey-golubev

llvm/include/llvm/ADT/STLExtras.h

@@ -1203,7 +1203,8 @@ class indexed_accessor_iterator
     return index - rhs.index;
   }
   bool operator==(const indexed_accessor_iterator &rhs) const {
-    return base == rhs.base && index == rhs.index;
+    assert(base == rhs.base && "incompatible iterators");
+    return index == rhs.index;
   }
   bool operator<(const indexed_accessor_iterator &rhs) const {
     assert(base == rhs.base && "incompatible iterators");