assert await precedence

[leobalter]

Capturing a convo from irc #tc39:

[17:55] <+ljharb> what's the correct precedence of await here? babel/minify#460 (comment)
[17:55] <+ljharb> is await a || b the same as (await a) || b, or await (a || b)?
[18:16] <+bradleymeck> ljharb: https://tc39.github.io/ecma262/#sec-unary-operators same level as delete / typeof
[19:15] <+ljharb> bradleymeck: ok, so that means the former?[19:20] <+bterlson> it's the former
[19:20] <+bterlson> yield precedence is the latter

I would like to investigate if we cover this precedence. If not, we should add a test for this.

cc @ljharb