When generating llvm.used, we may need an addrspacecast instead of a bitcast.

Summary:
This is especially important for targets that use multiple address spaces,
and commonly place global variables in address spaces other than zero.

Fixes PR22383

Test Plan: New test case added: llvm-used.cu

Reviewers: jingyue

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7345

llvm-svn: 227861