So far we didn't properly constant-fold JSToString operators in
JSTypedLowering where the input was a known number constant.

Bug: v8:6815
Change-Id: Iac87346b7d38f0f75461f285ea7daa2d5a5e1524
Reviewed-on: https://chromium-review.googlesource.com/663358Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47972}