Make it possible to assert that certain instance types are compatible wrt HAllocate.

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1753 matching lines @@
   // Check if we should create a cons string.
   IfBuilder if_createcons(this);
   if_createcons.If<HCompareNumericAndBranch>(
       length, Add<HConstant>(ConsString::kMinLength), Token::GTE);
   if_createcons.Then();
   {
     // Allocate the cons string object. HAllocate does not care whether we
     // pass CONS_STRING_TYPE or CONS_ASCII_STRING_TYPE here, so we just use
     // CONS_STRING_TYPE here. Below we decide whether the cons string is
     // one-byte or two-byte and set the appropriate map.
+    ASSERT(HAllocate::CompatibleInstanceTypes(CONS_STRING_TYPE,
+                                              CONS_ASCII_STRING_TYPE));
     HAllocate* string = Add<HAllocate>(Add<HConstant>(ConsString::kSize),
                                        HType::String(), pretenure_flag,
                                        CONS_STRING_TYPE);
 
     // Compute the intersection of instance types.
     HValue* anded_instance_types = AddUncasted<HBitwise>(
         Token::BIT_AND, left_instance_type, right_instance_type);
 
     // We create a one-byte cons string if
     // 1. both strings are one-byte, or
@@ skipping 9029 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal