Fix a bug in liveness analysis code and add more comments.

runtime/vm/bit_vector.h

Index: runtime/vm/bit_vector.h
diff --git a/runtime/vm/bit_vector.h b/runtime/vm/bit_vector.h
index cf692df85487f159ad1e48dca67d2f29d8775c80..81660b9246ee02ad285a6dc450f6b8f11196a7fa 100644
--- a/runtime/vm/bit_vector.h
+++ b/runtime/vm/bit_vector.h
@@ -64,32 +64,11 @@ class BitVector: public ZoneAllocated {
   }
 
   // Add all elements that are in the bitvector from.
-  bool AddAll(BitVector* from) {
-    ASSERT(data_length_ == from->data_length_);
-    bool changed = false;
-    for (intptr_t i = 0; i < data_length_; i++) {
-      const uword before = data_[i];
-      const uword after = data_[i] | from->data_[i];
-      if (before != after) changed = true;
-      data_[i] = after;
-    }
-    return changed;
-  }
+  bool AddAll(BitVector* from);
 
   // From the bitvector gen add those elements that are not in the
   // bitvector kill.
-  bool KillAndAdd(BitVector* kill, BitVector* gen) {
-    ASSERT(data_length_ == kill->data_length_);
-    ASSERT(data_length_ == gen->data_length_);
-    bool changed = false;
-    for (intptr_t i = 0; i < data_length_; i++) {
-      const uword before = data_[i];
-      const uword after = data_[i] | (gen->data_[i] & ~kill->data_[i]);
-      if (before != after) changed = true;
-      data_[i] = after;
-    }
-    return changed;
-  }
+  bool KillAndAdd(BitVector* kill, BitVector* gen);
 
   bool Contains(int i) const {
     ASSERT(i >= 0 && i < length());