[NVPTX] Annotate call machine instructions as calls.

Summary:
Otherwise we'll try to do unsafe optimizations on these MIs, such as
sinking loads below calls.

(I suspect that this is not the only bug in the NVPTX instruction
tablegen files; I need to comb through them.)

Reviewers: jholewinski, tra

Subscribers: jingyue, jhen, llvm-commits

Differential Revision: http://reviews.llvm.org/D17315

llvm-svn: 261113