[Attributor] Do not set 'returned' attribute for arguments that cannot be...

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

Reviewers: jdoerfert, sstefan1, uenoku

Reviewed By: jdoerfert

Subscribers: hiraditya, uenoku, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78828