This avoids write barrier when writing smis to tagged fields.

This includes writing to contexts, see example below:

var f = (function() {
  var i = 0;
  return function f() {
    return i++;  // Write barrier when writing to context.
  }
})();

f(); f(); %OptimizeFunctionOnNextCall(f);
f();

Review-Url: https://codereview.chromium.org/2925793002
Cr-Commit-Position: refs/heads/master@{#45723}