[DSE,MemorySSA] Check for underlying objects first.

isWriteAtEndOfFunction needs to check all memory uses of Def, which is
much more expensive than getting the underlying objects in practice.
Switch the call order, as recommended by the TODO, which was added as
per an earlier review.

This shaves off a bit of compile-time.

lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -2045,29 +2045,28 @@ struct DSEState {
           !isRemovable(Def->getMemoryInst()))
         continue;
 
-      // TODO: Consider doing the underlying object check first, if it is
-      // beneficial compile-time wise.
-      if (isWriteAtEndOfFunction(Def)) {
-        Instruction *DefI = Def->getMemoryInst();
-        // See through pointer-to-pointer bitcasts
-        SmallVector<const Value *, 4> Pointers;
-        getUnderlyingObjects(getLocForWriteEx(DefI)->Ptr, Pointers);
+      Instruction *DefI = Def->getMemoryInst();
+      SmallVector<const Value *, 4> Pointers;
+      auto DefLoc = getLocForWriteEx(DefI);
+      if (!DefLoc)
+        continue;
+      getUnderlyingObjects(DefLoc->Ptr, Pointers);
 
+      bool CanKill = true;
+      for (const Value *Pointer : Pointers) {
+        if (!InvisibleToCallerAfterRet.count(Pointer)) {
+          CanKill = false;
+          break;
+        }
+      }
+
+      if (CanKill && isWriteAtEndOfFunction(Def)) {
+        // See through pointer-to-pointer bitcasts
         LLVM_DEBUG(dbgs() << "   ... MemoryDef is not accessed until the end "
                              "of the function\n");
-        bool CanKill = true;
-        for (const Value *Pointer : Pointers) {
-          if (!InvisibleToCallerAfterRet.count(Pointer)) {
-            CanKill = false;
-            break;
-          }
-        }
-
-        if (CanKill) {
-          deleteDeadInstruction(DefI);
-          ++NumFastStores;
-          MadeChange = true;
-        }
+        deleteDeadInstruction(DefI);
+        ++NumFastStores;
+        MadeChange = true;
       }
     }
     return MadeChange;