Fix group barrier execution scope after sub-group feature is added

[adarshyoga]

The execution scope of group_barrier needs to be determined based on the group argument. Currently it is hard-coded to MemoryScope.WORK_GROUP since numba-dpex only supports group_barrier on work groups. Once sub-groups are added, then group_barrier can be called on both work groups and sub-groups.

The following changes need to performed once sub-group feature is added.

If group refers to a work_group then, execution scope needs to be set to MemoryScope.WORK_GROUP.
If group refers to a sub-group then, execution scope needs to be set to MemoryScope.SUB_GROUP.