Make optimize llvm common to both gpu-to-hsaco/cubin

Make optimize llvm common to both gpu-to-hsaco/cubin

Before serializing, optimizations on llvm were only called on path to
hsaco, and not cubin. Define opt-level for `gpu-to-cubin` pass as well,
and move call to optimize llvm to a common place.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D151554