[CUDA] Use activemask.b32 instruction to implement __activemask w/ CUDA-9.2+

vote.ballot instruction is gone in recent CUDA versions and
vote.sync.ballot can not be used because it needs a thread mask parameter.
Fortunately PTX 6.2 (introduced with CUDA-9.2) provides activemask.b32
instruction for this.

Differential Revision: https://reviews.llvm.org/D66665

llvm-svn: 370792