(WIP) lower waveid on GFX9

[zwu-2025]

There are still some problems in this draft so it's tagged with WIP

There is no safeway to get waveid, which is computed in getLaneAndWarpId
on GFX9 by reading a scalar register.
In this PR, we add a new intrinsic gfx9_wave_id with attribute convergent.
In the lowering phase, we will re-compute workitem.id and apply
waveid = (workitem.id.x & (num_warps * threads_per_warp)) / wavefront_size

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