[MemoryBuiltins] Fix an issue with hasNoAliasAttr

Summary:
`hasNoAliasAttr` is buggy: it checks to see if the called function has
a `noalias` attribute, which is incorrect since functions are not even
allowed to have the `noalias` attribute.  The comment on its only
caller, `llvm::isNoAliasFn`, makes it pretty clear that the intention
to do the `noalias` check on the return value, and not the callee.

Unfortunately I couldn't find a way to test this upstream -- fixing
this does not change the observable behavior of any of the passes that
use this.  This is not very surprising, since `noalias` does not tell
anything about the contents of the allocated memory (so, e.g., you
still cannot fold loads).  I'll be happy to be proven wrong though.

Reviewers: chandlerc, reames

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D17037

llvm-svn: 260298

llvm/lib/Analysis/MemoryBuiltins.cpp

@@ -144,7 +144,7 @@ static const AllocFnsTy *getAllocationData(const Value *V, AllocType AllocTy,
 
 static bool hasNoAliasAttr(const Value *V, bool LookThroughBitCast) {
   ImmutableCallSite CS(LookThroughBitCast ? V->stripPointerCasts() : V);
-  return CS && CS.hasFnAttr(Attribute::NoAlias);
+  return CS && CS.paramHasAttr(AttributeSet::ReturnIndex, Attribute::NoAlias);
 }