Tweak SmiAdd for X64

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 09dae3e09608b248886bd0a7b47a783a9171d25b..f393742ff9c56ba92cbfa0c6e6a40857e76057ab 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -578,8 +578,8 @@ class MacroAssembler: public Assembler {
               Label::Distance near_jump = Label::kFar);
 
   // Adds smi values and return the result as a smi.
-  // If dst is src1, then src1 will be destroyed, even if
-  // the operation is unsuccessful.
+  // If dst is src1, then src1 will be destroyed if the operation is
+  // successful, otherwise kept intact.
   void SmiAdd(Register dst,
               Register src1,
               Register src2,