perform_call() wants a ReadonlySpan<Value>, so just grab a slice of the
current register window instead of making a MarkedVector.
10% speed-up on this function call microbenchmark:
function callee(a, b, c) { }
function caller(callee) {
for (let i = 0; i < 10_000_000; ++i)
callee(1, 2, 3)
}
caller(callee)