[SelectionDAG][FIX] Allow "returned" arguments to be bit-casted

Summary:
An argument that is return by a function but bit-casted before can still
be annotated as "returned". Make sure we do not crash for this case.

Reviewers: sunfish, stephenwlin, niravd, arsenm

Subscribers: wdng, hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59917

llvm-svn: 362546