[NVPTX] Fix barrier.ll LIT test

The second parameter should be a multiple of the warp size (32).

PTX ISA spec, s9.7.12.1. Parallel Synchronization and Communication
Instructions: bar, barrier

barrier.sync{.aligned}      a{, b};

Operand b specifies the number of threads participating in the
barrier. If no thread count is specified, all threads in the CTA
participate in the barrier. When specifying a thread count, the value
must be a multiple of the warp size.

Differential Revision: https://reviews.llvm.org/D123470