[Beignet] [PATCH 3/5] add third coord in backend

[Xing, Homer]

[Gong, Zhigang] No, you don't have to create new functions. Just add one new parameter to specify the coords' dimension in IR layer, and specify the type in the extra.function in the gen instruction selection layer should be good enough.

We cannot add a new extra parameter to specify the coords' dimension in TypedWriteInstruction. That's the problem.
If you can solve it, please let me know.