sub_group_ballot_bit_count() and sub_group_ballot_find_msb() mask
their input according to a subgroup size, which is assumed to be the
maximum subgroup size, and not the actual subgroup size excluding
non-existent work-items in the "remainder" subgroup.
Fix this as per the the clarification made to the OpenCL C specification
in revision 3.0.9 for issue KhronosGroup/OpenCL-Docs#626 by pull request
KhronosGroup/OpenCL-Docs#689.
Signed-off-by: Stuart Brady <stuart.brady@arm.com>