[Inliner] Remove -update-return-attrs flag

[aeubanks]

Summary

This is by default on and I don't see any reason to turn it off. There's also no testing of it.

[aeubanks]

update

[nikic]

LG

llvm/lib/Transforms/Utils/InlineFunction.cpp

static AttrBuilder IdentifyValidAttributes(CallBase &CB) {
   if (AB.contains(Attribute::NoAlias))
     Valid.addAttribute(Attribute::NoAlias);
   if (AB.contains(Attribute::NonNull))
     Valid.addAttribute(Attribute::NonNull);
   return Valid;
 }
 
 static void AddReturnAttributes(CallBase &CB, ValueToValueMapTy &VMap) {
-  if (!UpdateReturnAttributes)
-    return;
-
   AttrBuilder Valid = IdentifyValidAttributes(CB);
   if (!Valid.hasAttributes())
     return;
   auto *CalledFunction = CB.getCalledFunction();
   auto &Context = CalledFunction->getContext();
 
   for (auto &BB : *CalledFunction) {
     auto *RI = dyn_cast<ReturnInst>(BB.getTerminator());