[compiler] Make Compiler::EnsureBytecode not switch tiers.

src/compiler.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler.h"
 
 #include <algorithm>
 #include <memory>
 
 #include "src/asmjs/asm-js.h"
@@ skipping 1374 matching lines @@
   // free as much as possible, since some code expects the old shared function
   // infos to stick around.
   script->set_shared_function_infos(*old_function_infos);
 
   return infos;
 }
 
 bool Compiler::EnsureBytecode(CompilationInfo* info) {
   DCHECK(ShouldUseIgnition(info));
   if (!info->shared_info()->HasBytecodeArray()) {
-    DCHECK(!info->shared_info()->is_compiled());
+    Handle<Code> original_code(info->shared_info()->code());
     if (GetUnoptimizedCode(info).is_null()) return false;
+    DCHECK(info->shared_info()->is_compiled());
+    if (original_code->kind() == Code::FUNCTION) {
+      // Generating bytecode will install the {InterpreterEntryTrampoline} as
+      // shared code on the function. To avoid an implicit tier down we restore
+      // original baseline code in case it existed beforehand.
+      info->shared_info()->ReplaceCode(*original_code);
+    }
   }
   DCHECK(info->shared_info()->HasBytecodeArray());
   return true;
 }
 
 // TODO(turbofan): In the future, unoptimized code with deopt support could
 // be generated lazily once deopt is triggered.
 bool Compiler::EnsureDeoptimizationSupport(CompilationInfo* info) {
   DCHECK_NOT_NULL(info->literal());
   DCHECK_NOT_NULL(info->scope());
@@ skipping 563 matching lines @@
     DCHECK(shared->is_compiled());
     function->set_literals(cached.literals);
   } else if (shared->is_compiled()) {
     // TODO(mvstanton): pass pretenure flag to EnsureLiterals.
     JSFunction::EnsureLiterals(function);
   }
 }
 
 }  // namespace internal
 }  // namespace v8