Optimize ~~(expr) in optimized code.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
===================================================================
--- src/hydrogen-instructions.cc	(revision 11385)
+++ src/hydrogen-instructions.cc	(working copy)
@@ -870,6 +870,17 @@
 }
 
 
+HValue* HBitNot::Canonicalize() {
+  // Optimize ~~x, a common pattern used for ToInt32(x).
+  if (value()->IsBitNot()) {
+    HValue* result = HBitNot::cast(value())->value();
+    ASSERT(result->representation().IsInteger32());
+    return result;
+  }
+  return this;
+}
+
+
 HValue* HAdd::Canonicalize() {
   if (!representation().IsInteger32()) return this;
   if (CheckUsesForFlag(kTruncatingToInt32)) ClearFlag(kCanOverflow);