ARB_gl_spirv mentions variables that don't exist in GLSL

[NicolBolas]

Specifically, it talks about gl_Vertex/InstanceIndex, rather than gl_Vertex/InstanceID. However much I might wish that we had the former instead of the latter, GLSL sadly does not. Well, except for the KHR_vulkan_glsl variation, but that's a different matter. Indeed, the ARB_gl_spirv extension specifically calls out how GLSL doesn't have these variables in its "Corresponding features not added to GLSL that GL_KHR_vulkan_glsl added" section.

[pdaniell-nv]

Thanks for the fix. Merged and closing issue.