fix crashes in elim-uncnstr2

This would crash before:
(declare-fun x () (_ BitVec 4))
(assert (not (bvule x #x1)))
(apply elim-uncnstr2)

That's because the index_set iterator was querying qtail to compute the end of the iteration
But the problem is that elim-uncnstr2 may add new fmls to the goal, as in this case.
The bvule is replaced with an 'or', but since it's negated, it turns into 2 goals
Solve the issue by freezing the qtail for the iteration loop.
This is the right behavior for elim-uncnstr2, as it can't rewrite exprs that haven't been analyzed before

@NikolajBjorner please check if this the right behavior for the other simplifiers. Thank you

src/ast/simplifiers/dependent_expr_state.h

@@ -116,20 +116,17 @@ class dependent_expr_simplifier {
     unsigned qtail() const { return m_fmls.qtail(); }
     struct iterator {
         dependent_expr_simplifier& s;
-        unsigned m_index = 0;
-        bool at_end = false;
-        unsigned index() const { return at_end ? s.qtail() : std::min(m_index, s.qtail()); }
-        iterator(dependent_expr_simplifier& s, unsigned i) : s(s), m_index(i), at_end(i == s.qtail()) {}
-        bool operator==(iterator const& other) const { return index() == other.index(); }
-        bool operator!=(iterator const& other) const { return !(*this == other); }
-        iterator& operator++() { if (!s.m.inc() || s.m_fmls.inconsistent()) at_end = true; else ++m_index; return *this; }
+        unsigned m_index, m_end;
+        iterator(dependent_expr_simplifier& s, unsigned i, unsigned end) : s(s), m_index(i), m_end(end) {}
+        bool operator!=(iterator const& other) const { return m_index != other.m_index; }
+        iterator& operator++() { if (!s.m.inc() || s.m_fmls.inconsistent()) m_index = m_end; else ++m_index; return *this; }
         unsigned operator*() const { return m_index; }
     };
 
     struct index_set {
         dependent_expr_simplifier& s;
-        iterator begin() { return iterator(s, s.qhead()); }
-        iterator end() { return iterator(s, s.qtail()); }
+        iterator begin() { return iterator(s, s.qhead(), s.qtail()); }
+        iterator end() { return iterator(s, s.qtail(), s.qtail()); }
         index_set(dependent_expr_simplifier& s) : s(s) {}
     };