[hydrogen] optimize switch with string clauses

src/ia32/lithium-codegen-ia32.h

Index: src/ia32/lithium-codegen-ia32.h
diff --git a/src/ia32/lithium-codegen-ia32.h b/src/ia32/lithium-codegen-ia32.h
index 1354e1d173e042eabfe3a0571f57d592303d72f7..ccbf0483713ab7b03a880651a918e0db69883598 100644
--- a/src/ia32/lithium-codegen-ia32.h
+++ b/src/ia32/lithium-codegen-ia32.h
@@ -280,6 +280,13 @@ 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 temp);