[NVPTX] Improve lowering of llvm.ctpop.

Summary:
Avoid an unnecessary conversion operation when using the result of
ctpop.i32 or ctpop.i16 as an i32, as in both cases the ptx instruction
we run returns an i32.

(Previously if we used the value as an i32, we'd do an unnecessary
zext+trunc.)

Reviewers: tra

Subscribers: jholewinski, llvm-commits

Differential Revision: https://reviews.llvm.org/D28721

llvm-svn: 292302