[crankshaft] Make sure +x is as fast as Number(x).

src/crankshaft/hydrogen-instructions.cc

Index: src/crankshaft/hydrogen-instructions.cc
diff --git a/src/crankshaft/hydrogen-instructions.cc b/src/crankshaft/hydrogen-instructions.cc
index bbe70874d79a76cbfac8feb21ccddbedbdf1ff5b..f21bc9a7ebd4a39c4abfa30d954f938c50c678ef 100644
--- a/src/crankshaft/hydrogen-instructions.cc
+++ b/src/crankshaft/hydrogen-instructions.cc
@@ -2958,6 +2958,8 @@ Maybe<HConstant*> HConstant::CopyToTruncatedNumber(Isolate* isolate,
     res = new (zone) HConstant(std::numeric_limits<double>::quiet_NaN());
   } else if (handle->IsNull()) {
     res = new(zone) HConstant(0);
+  } else if (handle->IsString()) {
+    res = new(zone) HConstant(String::ToNumber(Handle<String>::cast(handle)));
   }
   return res != NULL ? Just(res) : Nothing<HConstant*>();
 }