microsoft/compiler: zero out unused WebGPU system values
This patch zeros out first/baseVertex and baseInstance. We can do this because they should have already been hard-coded into vertex_id and instance_id by WebGPU's WGSL compiler. Also, WGSL does not have direct access to these system values.
If there is a need for standalone spriv-to-dxil, we can add an option to emit a UBO for them instead.