Discard is allowed in compute shader stage

[Noxime]

Description
A compute shader may include discard statements. This is not allowed by the WGSL spec.

Repro steps
Invoke a compute shader like

@compute @workgroup_size(1)
fn main(@builtin(global_invocation_id) global_id: vec3<u32>) {
    if global_id.x == 3u {
        discard;
    }
}

Expected vs observed behavior

• Expected: Shader is rejected
• Vulkan: Pipeline creation succeeds but validation layers catch the invalid use of OpKill. Device is lost upon dispatch.
• Dx12: Pipeline creation fails with an FXC error X4532

Extra materials
This can easily be reproduced by modifying the hello compute example

Platform
Windows 10, RTX 2070. Reproduced for wgpu 0.15.1 and #df54bc4