[ignition] Add DCHECK for FunctionLiteral scoping

This DCHECK would have triggered in the test cases in the attached bug.

Bug: chromium:740591
Change-Id: Ib8e866fe60f5f4ee825e6772f68be768925ed792
Reviewed-on: https://chromium-review.googlesource.com/585401
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46891}

src/interpreter/bytecode-generator.cc

@@ -1645,6 +1645,7 @@ void BytecodeGenerator::VisitDebuggerStatement(DebuggerStatement* stmt) {
 }
 
 void BytecodeGenerator::VisitFunctionLiteral(FunctionLiteral* expr) {
+  DCHECK_EQ(expr->scope()->outer_scope(), current_scope());
   uint8_t flags = CreateClosureFlags::Encode(
       expr->pretenure(), closure_scope()->is_function_scope());
   size_t entry = builder()->AllocateDeferredConstantPoolEntry();