[DSE] Don't check getUnderlyingObject() return value (NFC)

getUnderlyingObject() never returns null. It will simply return
something that is not the "root" underlying object.

Also drop a stale comment.

(cherry-picked from 14a49f5)

Author: nikic

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1268,10 +1268,6 @@ struct DSEState {
       if (CB->onlyAccessesInaccessibleMemory())
         return false;
 
-    // NOTE: For calls, the number of stores removed could be slightly improved
-    // by using AA.callCapturesBefore(UseInst, DefLoc, &DT), but that showed to
-    // be expensive compared to the benefits in practice. For now, avoid more
-    // expensive analysis to limit compile-time.
     return isRefSet(BatchAA.getModRefInfo(UseInst, DefLoc));
   }
 
@@ -1787,7 +1783,7 @@ struct DSEState {
       // uncommon. If it turns out to be important, we can use
       // getUnderlyingObjects here instead.
       const Value *UO = getUnderlyingObject(DefLoc->Ptr);
-      if (!UO || !isInvisibleToCallerAfterRet(UO))
+      if (!isInvisibleToCallerAfterRet(UO))
         continue;
 
       if (isWriteAtEndOfFunction(Def)) {
@@ -1916,9 +1912,7 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
     unsigned PartialLimit = MemorySSAPartialStoreLimit;
     // Worklist of MemoryAccesses that may be killed by KillingDef.
     SetVector<MemoryAccess *> ToCheck;
-
-    if (SILocUnd)
-      ToCheck.insert(KillingDef->getDefiningAccess());
+    ToCheck.insert(KillingDef->getDefiningAccess());
 
     bool Shortend = false;
     bool IsMemTerm = State.isMemTerminatorInst(SI);