[CUDA] Die gracefully when trying to output an LLVM alias.

Summary:
Previously, we would just output "foo = bar" in the assembly, and then
ptxas would choke.  Now we die before emitting any invalid code.

Reviewers: echristo

Subscribers: jholewinski, llvm-commits, jhen, tra

Differential Revision: http://reviews.llvm.org/D16490

llvm-svn: 258638