Remove redundant pointer map lookup

[mkuper]

It looks like this is on purpose:

https://reviews.llvm.org/rL68047
"getEntryFor() may invalidate DenseMap iterator.
Walking an invalidated iterator is not a good idea."

I think this is still true, but we should probably add a comment.

[mehdi_amini]

It'd be nice if the getEntryFor API provided a way for the caller to know if an insertion happened (i.e. using an API like insert()).

lib/Analysis/AliasSetTracker.cpp

   if (I == PointerMap.end())
     return;  // Noop
   assert(I->second->hasAliasSet() && "Dead entry?");
 
   AliasSet::PointerRec &Entry = getEntryFor(To);
   if (Entry.hasAliasSet()) return;    // Already in the tracker!
 
   // Add it to the alias set it aliases...
-  I = PointerMap.find_as(From);
   AliasSet *AS = I->second->getAliasSet(*this);
   AS->addPointer(*this, Entry, I->second->getSize(),
                  I->second->getAAInfo(),
                  true);
 }