Implement local check elimination on basic blocks.

src/unique.h

Index: src/unique.h
diff --git a/src/unique.h b/src/unique.h
index 68166541bce4ff796eb49eb788614def857c4457..30264ffb63afd8a7060896ad0b54b9fce2d069a1 100644
--- a/src/unique.h
+++ b/src/unique.h
@@ -162,6 +162,17 @@ class UniqueSet V8_FINAL : public ZoneObject {
     array_[size_++] = uniq;
   }
 
+  // Remove an element from this set. Mutates this set. O(|this|)
+  void Remove(Unique<T> uniq) {
+    for (int i = 0; i < size_; i++) {
+      if (array_[i] == uniq) {
+        while (++i < size_) array_[i - 1] = array_[i];
+        size_--;
+        return;
+      }
+    }
+  }
+
   // Compare this set against another set. O(|this|).
   bool Equals(UniqueSet<T>* that) const {
     if (that->size_ != this->size_) return false;
@@ -265,6 +276,10 @@ class UniqueSet V8_FINAL : public ZoneObject {
     return copy;
   }
 
+  void Clear() {
+    size_ = 0;
+  }
+
   inline int size() const {
     return size_;
   }