when merging two alias sets, the result set is volatile if either

of the sets is volatile.  We were dropping the volatile bit of the
merged in set, leading (luckily) to assertions in cases like 
PR7535.  I cannot produce a testcase that repros with opt, but this
is obviously correct.

llvm-svn: 112402

llvm/lib/Analysis/AliasSetTracker.cpp

@@ -35,6 +35,7 @@ void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST) {
   // Update the alias and access types of this set...
   AccessTy |= AS.AccessTy;
   AliasTy  |= AS.AliasTy;
+  Volatile |= AS.Volatile;
 
   if (AliasTy == MustAlias) {
     // Check that these two merged sets really are must aliases.  Since both