Fix PR33878: BasicAA incorrectly assumes different address spaces don't alias

[nlopes]

Summary

This fixes http://llvm.org/PR33878
This code assumes that a nullptr of address space != 0 cannot alias with a non-null pointer. This is incorrect, since nothing can be concluded about a null pointer in an address space != 0.
I trade down this code to revision r52299 (2008), before address spaces were introduced.

[hfinkel]

LGTM

[davide]

lg, thanks.

BasicAliasAnalysis.cpp

if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
       return NoAlias;
 
     // Function arguments can't alias with things that are known to be
     // unambigously identified at the function level.
     if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
         (isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
       return NoAlias;
 
-    // Most objects can't alias null.
-    if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
-        (isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
-      return NoAlias;
-
     // If one pointer is the result of a call/invoke or load and the other is a
     // non-escaping local object within the same function, then we know the
     // object couldn't escape to a point where the call could return it.
     //
     // Note that if the pointers are in different functions, there are a
     // variety of complications. A call with a nocapture argument may still
     // temporary store the nocapture argument's value in a temporary memory
     // location if that memory location doesn't escape. Or it may pass a