[MemorySSA] Fix checkClobberSanity to skip Start only for Defs and Uses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340981 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/MemorySSA.cpp

@@ -429,18 +429,23 @@ checkClobberSanity(const MemoryAccess *Start, MemoryAccess *ClobberAt,
       // We should never hit liveOnEntry, unless it's the clobber.
       assert(!MSSA.isLiveOnEntryDef(MA) && "Hit liveOnEntry before clobber?");
 
-      // If Start is a Def, skip self.
-      if (MA == Start)
-        continue;
-
       if (const auto *MD = dyn_cast<MemoryDef>(MA)) {
-        (void)MD;
+        // If Start is a Def, skip self.
+        if (MD == Start)
+          continue;
+
         assert(!instructionClobbersQuery(MD, MAP.second, Query.Inst, AA)
                     .IsClobber &&
                "Found clobber before reaching ClobberAt!");
         continue;
       }
 
+      if (const auto *MU = dyn_cast<MemoryUse>(MA)) {
+        assert (MU == Start &&
+                "Can only find use in def chain if Start is a use");
+        continue;
+      }
+
       assert(isa<MemoryPhi>(MA));
       Worklist.append(
           upward_defs_begin({const_cast<MemoryAccess *>(MA), MAP.second}),