Improve redundant load elimination

runtime/vm/bit_vector.cc

Index: runtime/vm/bit_vector.cc
diff --git a/runtime/vm/bit_vector.cc b/runtime/vm/bit_vector.cc
index c08eaf1d4de538d751c80574782514bdaae42923..80b640b247c38304c234c041209f2aed873e3e1f 100644
--- a/runtime/vm/bit_vector.cc
+++ b/runtime/vm/bit_vector.cc
@@ -62,6 +62,18 @@ bool BitVector::AddAll(const BitVector* from) {
 }
 
 
+bool BitVector::RemoveAll(const 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 BitVector::KillAndAdd(BitVector* kill, BitVector* gen) {
   ASSERT(data_length_ == kill->data_length_);
   ASSERT(data_length_ == gen->data_length_);