[ast] Simplify NewYieldStar.

yield* always has an argument.

R=rmcilroy@chromium.org

Bug: 
Change-Id: I5d14c0db05b1e1b873831e0f5a18ec479c1399c9
Reviewed-on: https://chromium-review.googlesource.com/566816Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46564}

src/ast/ast.h

@@ -3493,7 +3493,7 @@ class AstNodeFactory final BASE_EMBEDDED {
   }
 
   YieldStar* NewYieldStar(Expression* expression, int pos, SuspendFlags flags) {
-    if (!expression) expression = NewUndefinedLiteral(pos);
+    DCHECK_NOT_NULL(expression);
     return new (zone_) YieldStar(expression, pos, flags);
   }