Avoid unnecessary recursion when loop iterations don't invoke await

[MatAtBread]

As in the following case, where the loop rarely awaits.

async function x() {
    for (let i = 0; i < 1000000; i++) {
        if (i == 999999) await a;
    }
}

See microsoft/TypeScript#1664 (comment)

[matAtWork]

Implemented in https://github.com/MatAtBread/nodent/tree/acorn-4

Confirming compliance and performamce tests