[hydrogen] optimize switch with string clauses

src/x64/lithium-codegen-x64.h

Index: src/x64/lithium-codegen-x64.h
diff --git a/src/x64/lithium-codegen-x64.h b/src/x64/lithium-codegen-x64.h
index f3cb667973ce58c486e85c1ff752f09f34e666a8..0078a30c243ef2c083bfd717031d5773e44264cf 100644
--- a/src/x64/lithium-codegen-x64.h
+++ b/src/x64/lithium-codegen-x64.h
@@ -271,6 +271,13 @@ class LCodeGen BASE_EMBEDDED {
                          Label* is_not_object,
                          Label* is_object);
 
+  // Emits optimized code for %_IsString(x).  Preserves input register.
+  // Returns the condition on which a final split to
+  // true and false label should be made, to optimize fallthrough.
+  Condition EmitIsString(Register input,
+                         Label* is_not_object,
+                         Label* is_object);
+
   // Emits optimized code for %_IsConstructCall().
   // Caller should branch on equal condition.
   void EmitIsConstructCall(Register temp);