Refactor nxt function for clarity and efficiency

Author: lrvideckis

library/data_structures_[l,r]/seg_tree.hpp

@@ -11,8 +11,8 @@
 //! @time O(n + q log n)
 //! @space O(n)
 int nxt(int& l, int r) {
-  int x = __lg(min(l & -l, r - l + 1));
-  return ((l += (1 << x)) >> x) - 1;
+  int x = __lg(min(l & -l, r - l + 1)), y = l;
+  return l += (1 << x), y >> x;
   //return exchange(l, l + (1 << x)) >> x;
 }
 template<class T, class F> struct tree {