Add AsyncAwait opcode to speed up await code path

The logic for await expr is:

  - check if expr is null, if so return null
  - if expr is not a WaitHandle, call getWaitHandle

  - if the wait handle is finished, either pull out its result or
    throw its exception
  - else if it's unfinished, either suspend the continuation or if
    we're eagerly executing the async function, wrap it in a result
    wait handle and create an async function that will continue here.

This diff changes the null check to not do reference counting (use an
unnamed local instead of Dup), and makes an opcode to speed up the
logic in the second part, by removing function calls and reference
counting due to another Dup.

Reviewed By: @jano

Differential Revision: D1058415
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