This is a reland of ccbbdb93.

Original change's description:
> [turbofan] Disable speculation for JSCall nodes by default
>
> Change-Id: I7360601f4e1b419cf8d35480b068418bdd700be9
> Reviewed-on: https://chromium-review.googlesource.com/928649
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51467}

Bug: v8:7340

Change-Id: I5557afcdad0c7f9610a396dcfa45f8985a13c1ba
Reviewed-on: https://chromium-review.googlesource.com/931546Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51496}