Revert "clang tot bots: Force-enable memoryssa-based dse"

This reverts commit a359d11.

Reason for revert: Suspicious to be causing test failures on several bots.  Examples are:
https://ci.chromium.org/p/chromium/builders/ci/Cast%20Audio%20Linux
https://ci.chromium.org/p/chromium/builders/ci/Mac10.10%20Tests

Original change's description:
> clang tot bots: Force-enable memoryssa-based dse
> 
> Now that the flag is off-by-default again upstream, let's force it on
> on the tot bots to see if things work with it on.
> 
> TBR=akhuang
> 
> Bug: 1127713
> Change-Id: Ib9267b5123472b1c28280d858da7229b140a3b7e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415222
> Reviewed-by: Nico Weber <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#807727}

[email protected],[email protected]

Change-Id: I794de306768b4415f5404cacab6a69862d4f81e1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1127713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413686
Reviewed-by: Yuki Shiino <[email protected]>
Commit-Queue: Yuki Shiino <[email protected]>
Cr-Commit-Position: refs/heads/master@{#807745}

build/config/compiler/BUILD.gn

@@ -505,7 +505,7 @@ config("compiler") {
       cflags += [
         # TODO(https://crbug.com/1127713): Investigate, remove.
         "-mllvm",
-        "-enable-dse-memoryssa=true",
+        "-enable-dse-memoryssa=false",
       ]
     }
   }