This reverts commit ccbbdb93.

Reason for revert: https://logs.chromium.org/v/?s=chromium%2Fbb%2Fclient.v8%2FV8_Linux%2F23316%2F%2B%2Frecipes%2Fsteps%2FBisect_50a6e301%2F0%2Fsteps%2FRetry_-_nosse3%2F0%2Flogs%2FJSCallReducerTest.Mat..%2F0

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}

TBR=sigurds@chromium.org,bmeurer@chromium.org

Change-Id: I5a1988902730be9e962b17a434251db82d834b98
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/931401Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51471}