Fix improper usage of SmiUntag operation in CodeStubAssembler.

src/code-stub-assembler.cc

Index: src/code-stub-assembler.cc
diff --git a/src/code-stub-assembler.cc b/src/code-stub-assembler.cc
index e5041756644d52698ce5b3702f1150397afcb544..c85be9b46e7b866b9025fcfdbb0b55dd09956094 100644
--- a/src/code-stub-assembler.cc
+++ b/src/code-stub-assembler.cc
@@ -259,7 +259,7 @@ Node* CodeStubAssembler::SmiToWord32(Node* value) {
 }
 
 Node* CodeStubAssembler::SmiToFloat64(Node* value) {
-  return ChangeInt32ToFloat64(SmiUntag(value));
+  return ChangeInt32ToFloat64(SmiToWord32(value));
 }
 
 Node* CodeStubAssembler::SmiAdd(Node* a, Node* b) { return IntPtrAdd(a, b); }