Common: Simplify and optimize BitSet iterator

Instead of doing complex machinations to keep track of the current bit
index, just unset the lowest bit on each iteration, greatly simplifying
the code.

src/common/bit_set.h

@@ -120,20 +120,15 @@ public:
     // A STL-like iterator is required to be able to use range-based for loops.
     class Iterator {
     public:
-        Iterator(const Iterator& other) : m_val(other.m_val), m_bit(other.m_bit) {}
-        Iterator(IntTy val) : m_val(val), m_bit(0) {}
-        Iterator& operator=(Iterator other) {
-            new (this) Iterator(other);
-            return *this;
-        }
+        Iterator(const Iterator& other) : m_val(other.m_val) {}
+        Iterator(IntTy val) : m_val(val) {}
         int operator*() {
-            return m_bit + ComputeLsb();
+            // This will never be called when m_val == 0, because that would be the end() iterator
+            return LeastSignificantSetBit(m_val);
         }
         Iterator& operator++() {
-            int lsb = ComputeLsb();
-            m_val >>= lsb + 1;
-            m_bit += lsb + 1;
-            m_has_lsb = false;
+            // Unset least significant set bit
+            m_val &= m_val - IntTy(1);
             return *this;
         }
         Iterator operator++(int _) {
@@ -149,17 +144,7 @@ public:
         }
 
     private:
-        int ComputeLsb() {
-            if (!m_has_lsb) {
-                m_lsb = LeastSignificantSetBit(m_val);
-                m_has_lsb = true;
-            }
-            return m_lsb;
-        }
         IntTy m_val;
-        int m_bit;
-        int m_lsb = -1;
-        bool m_has_lsb = false;
     };
 
     BitSet() : m_val(0) {}