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

This reverts commit a173775c.

Reason for revert: Broken builders, e.g.:

https://ci.chromium.org/p/chromium/builders/ci/GPU%20FYI%20Win%20dEQP%20Builder/100576
https://ci.chromium.org/p/chromium/builders/ci/win-swangle-x86/13670

Original change's description:
> Reland "clang tot bots: Force-enable memoryssa-based dse"
> 
> This is a reland of a359d11d
> Fix conditional to enable this on the tot bots, not on the non-tot bots.
> 
> 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 <thakis@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#807727}
> 
> Bug: 1127713
> Change-Id: I1252ee68251279c82da5ef00c06f7f1514780baa
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416668
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#807882}

TBR=thakis@chromium.org,akhuang@google.com

Change-Id: I45874319ef7c4643909694a9f5e138cf693f9837
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1127713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416750Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807897}

build/config/compiler/BUILD.gn

@@ -501,11 +501,11 @@ config("compiler") {
       "-instcombine-lower-dbg-declare=0",
     ]
 
-    if (llvm_force_head_revision) {
+    if (!llvm_force_head_revision) {
       cflags += [
         # TODO(https://crbug.com/1127713): Investigate, remove.
         "-mllvm",
-        "-enable-dse-memoryssa=true",
+        "-enable-dse-memoryssa=false",
       ]
     }
   }