[Attributor] Do not set 'returned' attribute for arguments that cannot be bitcasted to function result

[jdoerfert]

LGTM. Thx!

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

if (CB.getNumUses() == 0)
       return ChangeStatus::UNCHANGED;
     if (A.changeValueAfterManifest(CB, C))
       return ChangeStatus::CHANGED;
     return ChangeStatus::UNCHANGED;
   };
 
   // If the assumed unique return value is an argument, annotate it.
   if (auto *UniqueRVArg = dyn_cast<Argument>(UniqueRV.getValue())) {
+    if (UniqueRVArg->getType()->canLosslesslyBitCastTo(
+            getAssociatedFunction()->getReturnType())) {
       // TODO: This should be handled differently!
       this->AnchorVal = UniqueRVArg;
       this->KindOrArgNo = UniqueRVArg->getArgNo();
       Changed = IRAttribute::manifest(A);
+    }
   } else if (auto *RVC = dyn_cast<Constant>(UniqueRV.getValue())) {
     // We can replace the returned value with the unique returned constant.
     Value &AnchorValue = getAnchorValue();
     if (Function *F = dyn_cast<Function>(&AnchorValue)) {
       for (const Use &U : F->uses())
         if (CallBase *CB = dyn_cast<CallBase>(U.getUser()))
           if (CB->isCallee(&U)) {
             Constant *RVCCast =

llvm/test/Transforms/Attributor/returned_crash.ll

+; RUN: opt -attributor -S %s | FileCheck %s
+; RUN: opt -passes=attributor -S %s | FileCheck %s
+;
+; CHECK: define i32 addrspace(1)* @foo(i32 addrspace(4)* nofree readnone %arg)
+define i32 addrspace(1)* @foo(i32 addrspace(4)* %arg) {
+entry:
+  %0 = addrspacecast i32 addrspace(4)* %arg to i32 addrspace(1)*
+  ret i32 addrspace(1)* %0
+}