[bigint,compiler] Further refine some typings.

Bug: v8:6791
Change-Id: I31acb7f24323f544a2930e6338a178ac66806ea9
Reviewed-on: https://chromium-review.googlesource.com/829134Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50151}

src/compiler/typer.cc

@@ -421,7 +421,9 @@ Type* Typer::Visitor::BinaryNumberOpTyper(Type* lhs, Type* rhs, Typer* t,
   if (lhs_is_number || rhs_is_number) {
     return Type::Number();
   }
-  // TODO(neis): Check if one side is BigInt in order to return BigInt?
+  if (lhs->Is(Type::BigInt()) || rhs->Is(Type::BigInt())) {
+    return Type::BigInt();
+  }
   return Type::Numeric();
 }