clang: Re-enable dse memory ssa on trunk bots.

Else we can't tell if it's working on trunk.

TBR=rnk

Bug: 1127713
Change-Id: I9cefb00517ea71399e382678ce6da81e3629b501
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410817Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806705}

build/config/compiler/BUILD.gn

@@ -501,11 +501,13 @@ config("compiler") {
       "-instcombine-lower-dbg-declare=0",
     ]
 
-    cflags += [
-      # TODO(https://crbug.com/1127713): Investigate, remove.
-      "-mllvm",
-      "-enable-dse-memoryssa=false",
-    ]
+    if (!llvm_force_head_revision) {
+      cflags += [
+        # TODO(https://crbug.com/1127713): Investigate, remove.
+        "-mllvm",
+        "-enable-dse-memoryssa=false",
+      ]
+    }
   }
 
   # C11/C++11 compiler flags setup.