thread, grid, and block dim/idx can only return non-negative values

[ZuseZ4]

Based on the previous discussion in https://rust-lang.zulipchat.com/#narrow/channel/422870-t-compiler.2Fgpgpu-backend/topic/Return.20type.20of.20NVPTX.20index.20and.20dimension.20intrinsics

r? @workingjubilee