[turbofan] Ship generators if --turbo-from-bytecode set.

This makes sure generator functions are marked as optimizable for all
configurations where the BytecodeGraphBuilder is used. Note that as
usual AstNumbering is just a heuristic and the underlying compiler can
still bailout from optimization when the compilation pipeline chooses
another compiler that does not support generator functions.

R=bmeurer@chromium.org,hablich@chromium.org

Review-Url: https://codereview.chromium.org/2353793003
Cr-Commit-Position: refs/heads/master@{#39553}

src/ast/ast-numbering.cc

@@ -592,10 +592,12 @@ bool AstNumberingVisitor::Renumber(FunctionLiteral* node) {
   }
 
   if (IsGeneratorFunction(node->kind()) || IsAsyncFunction(node->kind())) {
-    // TODO(neis): We may want to allow Turbofan optimization here if
-    // --turbo-from-bytecode is set and we know that Ignition is used.
-    // Unfortunately we can't express that here.
-    DisableOptimization(kGenerator);
+    // Generators can be optimized if --turbo-from-bytecode is set.
+    if (FLAG_turbo_from_bytecode) {
+      DisableCrankshaft(kGenerator);
+    } else {
+      DisableOptimization(kGenerator);
+    }
   }
 
   VisitDeclarations(scope->declarations());