[GC][InstCombine] Fix a potential iteration issue

Noticed via inspection.  Appears to be largely innocious in practice, but slight code change could have resulted in either visit order dependent missed optimizations or infinite loops.  May be a minor compile time problem today.

llvm-svn: 346698

Author: preames

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -3902,8 +3902,11 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
         return replaceInstUsesWith(*II, ConstantPointerNull::get(PT));
 
       // isKnownNonNull -> nonnull attribute
-      if (isKnownNonZero(DerivedPtr, DL, 0, &AC, II, &DT))
+      if (!II->hasRetAttr(Attribute::NonNull) &&
+          isKnownNonZero(DerivedPtr, DL, 0, &AC, II, &DT)) {
         II->addAttribute(AttributeList::ReturnIndex, Attribute::NonNull);
+        return II;
+      }
     }
 
     // TODO: bitcast(relocate(p)) -> relocate(bitcast(p))