This makes the context allocation less pessimistic in the following cases:

function outer() {
  var a; // Won't be context allocated
  function inner1() { var a; a; }
  function inner2(a) { a; }
  function inner3([a]) { a; }
  function inner4({ a: b}) { a; }
}

BUG=v8:5501

Review-Url: https://codereview.chromium.org/2407163003
Cr-Commit-Position: refs/heads/master@{#41521}