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

test/mjsunit/regress/regress-crbug-635923.js

+// Copyright 2016 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.
+
+// Flags: --allow-natives-syntax --ignition --turbo-from-bytecode --turbo-filter=f
+
+function f(x) { return x + 23 }
+function g(x) { return f(x) + 42 }
+
+assertEquals(23, f(0));
+assertEquals(24, f(1));
+assertEquals(67, g(2));
+assertEquals(68, g(3));
+
+// Optimize {g} with Crankshaft, causing {f} to be inlined.
+%OptimizeFunctionOnNextCall(g);
+assertEquals(65, g(0));
+
+// Optimize {f} with Turbofan, after it has been inlined.
+%OptimizeFunctionOnNextCall(f);
+assertEquals(23, f(0));