[NVPTX] Check if callsite is defined when computing argument allignment

Summary: In getArgumentAlignment check if the ImmutableCallSite pointer CS is non-null before dereferencing. If CS is 0x0 fall back to the ABI type alignment else compute the alignment as before.

Reviewers: eliben, jpienaar

Subscribers: jlebar, vchuravy, cfe-commits, jholewinski

Differential Revision: https://reviews.llvm.org/D9168

llvm-svn: 282045