Workgroup size from spec constant

When you use subgroups, you can often simplify code by assuming subgroup size == workgroup size for any subgroup intrinsics. But subgroup size is defined by the HW you are executing on, so what you'd need to do is use a specialization constant to set your workgroup size. You can already do this in glsl since vulkan first released:

The built-in vector gl_WorkGroupSize can be specialized using special layout local_size_{xyz}_id's applied to the "in" qualifier. For example:
layout(local_size_x_id = 18, local_size_z_id = 19) in;
This leaves gl_WorkGroupSize.y as a non-specialization constant, with gl_WorkGroupSize being a partially specialized vector. Its x and z components can be later specialized using the ID's 18 and 19.

https://github.com/KhronosGroup/GLSL/blob/3e0d9a3b3f54651ef53d533392a7401a3b19e16d/extensions/khr/GL_KHR_vulkan_glsl.txt#L192

Start with the linked GL_KHR_vulkan_glsl specification passage describing specialization constants for gl_WorkGroupSize, then locate the Rust-GPU compiler path that handles workgroup size and specialization constants. Done means subgroup-related workgroup size assumptions can use a specialization constant, with coverage added for the supported behavior.