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/+/2416668Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807882}

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=false",
+        "-enable-dse-memoryssa=true",
       ]
     }
   }