[hydrogen] optimize switch with string clauses

src/arm/lithium-codegen-arm.h

Index: src/arm/lithium-codegen-arm.h
diff --git a/src/arm/lithium-codegen-arm.h b/src/arm/lithium-codegen-arm.h
index b01e4967d77a62f7baf45d7905abfe1ddfd815d3..c84391ac37f97ccd4ab548e602eaa2e184ba2c12 100644
--- a/src/arm/lithium-codegen-arm.h
+++ b/src/arm/lithium-codegen-arm.h
@@ -285,6 +285,13 @@ 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,
+                         Register temp1,
+                         Label* is_not_string);
+
   // Emits optimized code for %_IsConstructCall().
   // Caller should branch on equal condition.
   void EmitIsConstructCall(Register temp1, Register temp2);