[Beignet] [PATCH 6/7] Support global constant arrays

[Xing, Homer]

Yes, I think so. Will send version two of this patch.

> Only one comment here, if the con.getOffset() is zero, then we can save one instruction.
> And furthermore, if the offset is zero, can we just set the reg as ir::ocl::constoffst's proxy register, then we can save two instructions here? I'm not very sure of this. But at least, you can save at least one instruction if the offset is zero. For all the rest of the patch, LGTM. Thanks for the patch.