[crankshaft] Re-add fast-case for string add left/right.

src/code-stubs.h

Index: src/code-stubs.h
diff --git a/src/code-stubs.h b/src/code-stubs.h
index 20da57f0214b834d10f009fd628767089eacd955..4b259b6de0bee0f2d2774407ee91be714a73ddf2 100644
--- a/src/code-stubs.h
+++ b/src/code-stubs.h
@@ -676,7 +676,11 @@ enum StringAddFlags {
   // Check right parameter.
   STRING_ADD_CHECK_RIGHT = 1 << 1,
   // Check both parameters.
-  STRING_ADD_CHECK_BOTH = STRING_ADD_CHECK_LEFT | STRING_ADD_CHECK_RIGHT
+  STRING_ADD_CHECK_BOTH = STRING_ADD_CHECK_LEFT | STRING_ADD_CHECK_RIGHT,
+  // Convert parameters when check fails (instead of throwing an exception).
+  STRING_ADD_CONVERT = 1 << 2,
+  STRING_ADD_CONVERT_LEFT = STRING_ADD_CHECK_LEFT | STRING_ADD_CONVERT,
+  STRING_ADD_CONVERT_RIGHT = STRING_ADD_CHECK_RIGHT | STRING_ADD_CONVERT
 };
 
 
@@ -701,8 +705,8 @@ class StringAddTFStub : public TurboFanCodeStub {
   }
 
  private:
-  class StringAddFlagsBits : public BitField<StringAddFlags, 0, 2> {};
-  class PretenureFlagBits : public BitField<PretenureFlag, 2, 1> {};
+  class StringAddFlagsBits : public BitField<StringAddFlags, 0, 3> {};
+  class PretenureFlagBits : public BitField<PretenureFlag, 3, 1> {};
 
   void PrintBaseName(std::ostream& os) const override;  // NOLINT
 
@@ -1647,8 +1651,8 @@ class StringAddStub final : public HydrogenCodeStub {
   static const int kRight = 1;
 
  private:
-  class StringAddFlagsBits: public BitField<StringAddFlags, 0, 2> {};
-  class PretenureFlagBits: public BitField<PretenureFlag, 2, 1> {};
+  class StringAddFlagsBits : public BitField<StringAddFlags, 0, 3> {};
+  class PretenureFlagBits : public BitField<PretenureFlag, 3, 1> {};
 
   void PrintBaseName(std::ostream& os) const override;  // NOLINT