The JSCreateClosure operator was not marked as Eliminatable, esp. it
wasn't marked as NoWrite (read: no JavaScript observable side-effect),
which lead to a weird performance cliff with the new Array builtin
inlining. For example

  a.forEach(c => c);

was not inlined, whereas

  const f = c => c;
  a.forEach(f);

was properly inlined, despite not causing any trouble for TurboFan in
general. The reason was that the JSCreateClosure for the arrow function
was marked as "can cause potential side effect", which it cannot. This
fixes the operator to be properly marked as Eliminatable, thus removing
this performance cliff.

BUG=v8:1956,v8:6475
R=danno@chromium.org

Review-Url: https://codereview.chromium.org/2930933002
Cr-Commit-Position: refs/heads/master@{#45801}