GEP not implemented for type struct [_; 0]

Original issue: https://github.com/EmbarkStudios/rust-gpu/issues/1075

Expected Behaviour

This compiles fine with rustc (I guess this bit is probably eliminated?).

Example & Steps To Reproduce

I was using const generic parameters to determine the length of some arrays. At some point, a base case ended up declaring a zero-length array, like so:

#![cfg_attr(target_arch = "spirv", no_std)]
use spirv_std::spirv;

fn example<const LENGTH: usize>() {
    let mut array = [0; LENGTH];
    for i in 0..array.len() {
        array[i] += i;
    }
}

#[spirv(compute(threads(1, 1, 1)))]
pub fn compute() {
    example::<0>();
}

Resulting in:

error: GEP not implemented for type struct [usize; 0] {  }
 --> examples/shaders/compute-shader/src/lib.rs:7:9
  |
7 |         array[i] += i;
  |         ^^^^^^^^^^^^^

System Info

* rustc: 1.71.0-nightly (1a5f8bce7 2023-05-26)

* rust-gpu: [b2e5eb7](https://github.com/EmbarkStudios/rust-gpu/commit/b2e5eb75951545782b121665c20f9f1c66fe5bdd)

Start by reproducing the error from examples/shaders/compute-shader/src/lib.rs with the zero-length const-generic array and confirm the GEP failure. Trace the compiler path handling indexing of the reported struct [usize; 0] type. Done means the example compiles successfully without the GEP error and the behavior is covered by a regression test.