nir/lower_subgroups: Pad ballot values before bitcasting
Otherwise, if we cast from a uint32_t to a uint64_t, the bitcast will fail before we pad. This happens on Intel.
Fixes: e4e79de2 "nir/subgroups: Support > 1 ballot components"
Fixes this fallout: http://mesa-ci-results.jf.intel.com/mesa_master/builds/27058/group/63a9f0ea7bb98050796b649e85481845