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

@@ -501,12 +501,14 @@ config("compiler") {
       "-instcombine-lower-dbg-declare=0",
     ]
 
+    if (!llvm_force_head_revision) {
       cflags += [
         # TODO(https://crbug.com/1127713): Investigate, remove.
         "-mllvm",
         "-enable-dse-memoryssa=false",
       ]
     }
+  }
 
   # C11/C++11 compiler flags setup.
   # ---------------------------