This reverts commit 86bc1517.

Reason for revert: breaks gcc bot:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20gcc%204.8/builds/17209

Original change's description:
> [wasm] remove kExecuteSimdLowered mode from wasm cctest
> 
> R=​clemensh@chromium.org,titzer@chromium.org,bbudge@chromium.org,gdeepti@chromium.org
> BUG=v8:7028
> 
> Change-Id: Ie0b984ebd18e267cdaf7aaff9f17fb4328d8e5fa
> Reviewed-on: https://chromium-review.googlesource.com/849638
> Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50385}

TBR=bbudge@chromium.org,titzer@chromium.org,gdeepti@chromium.org,aseemgarg@chromium.org,ahaas@chromium.org,clemensh@chromium.org

Change-Id: I890b8810ea802fe2b9273def07c9056d4b904a4e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7028
Reviewed-on: https://chromium-review.googlesource.com/852712Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50386}