Add guard against self assignment.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index e2fbfb59c4b6fbf3462ba1343b7e8fcb1354997e..944a501bd92067057a2c7347de5ec992aef7daf3 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1364,7 +1364,9 @@ HSideEffectMap::HSideEffectMap(HSideEffectMap* other) : count_(other->count_) {
 
 
 HSideEffectMap& HSideEffectMap::operator= (const HSideEffectMap& other) {
-  memcpy(data_, other.data_, kNumberOfTrackedSideEffects * kPointerSize);
+  if (this != &other) {
+    memcpy(data_, other.data_, kNumberOfTrackedSideEffects * kPointerSize);
+  }
   return *this;
 }