This CL introduces type narrowing and constant folding reducers
to constant fold code that comes out of inlined destructuring
of arrays. In particular, array iterator introduces code that
contains a phi of a temporary array that blocks escape analysis.
The phi comes from conditional that can be evaluated statically
(i.e., constant folded), so with better constant folding we
allow escape analysis to get rid of the temporary array.

On a quick micro-benchmark below, we see more than 6x improvement.
This is close to the hand-optimized version - if we replace
body of f with 'return b + a', we get 220ms (versus 218ms with
destructuring).

function f(a, b) {
  [b, a] = [a, b];
  return a + b;
}

function sum(count) {
  let s = 0;
  for (let i = 0; i < count; i++) {
    s += f(1, 2);
  }
  return s;
}

// Warm up
sum(1e5); sum(1e5);
console.time("destructure array");
sum(1e8);
console.timeEnd("destructure array");

console.timeEnd: destructure array, 213.526000

console.timeEnd: destructure array, 1503.537000

Bug: v8:7728
Change-Id: Ib7aec1d5897989e6adb1af1eddd516d8b3866db5
Reviewed-on: https://chromium-review.googlesource.com/1047672Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53048}