naga: Don't consider per-polygon inputs to be subgroup uniform

Implementations can absolutely pack multiple triangles per subgroup.

Fixes #6270.

CHANGELOG.md

@@ -93,6 +93,7 @@ By @bradwerth [#6216](https://github.com/gfx-rs/wgpu/pull/6216).
 
 - Accept only `vec3` (not `vecN`) for the `cross` built-in. By @ErichDonGubler in [#6171](https://github.com/gfx-rs/wgpu/pull/6171).
 - Configure `SourceLanguage` when enabling debug info in SPV-out. By @kvark in [#6256](https://github.com/gfx-rs/wgpu/pull/6256)
+- Per-polygon and flat inputs should not be considered subgroup uniform. By @magcius in [#6276](https://github.com/gfx-rs/wgpu/pull/6276).
 
 #### General
 

naga/src/valid/analyzer.rs

@@ -589,23 +589,16 @@ impl FunctionInfo {
                     requirements: UniformityRequirements::empty(),
                 }
             }
-            // depends on the builtin or interpolation
+            // depends on the builtin
             E::FunctionArgument(index) => {
                 let arg = &resolve_context.arguments[index as usize];
                 let uniform = match arg.binding {
                     Some(crate::Binding::BuiltIn(
-                        // per-polygon built-ins are uniform
-                        crate::BuiltIn::FrontFacing
                         // per-work-group built-ins are uniform
                         | crate::BuiltIn::WorkGroupId
                         | crate::BuiltIn::WorkGroupSize
                         | crate::BuiltIn::NumWorkGroups)
                     ) => true,
-                    // only flat inputs are uniform
-                    Some(crate::Binding::Location {
-                        interpolation: Some(crate::Interpolation::Flat),
-                        ..
-                    }) => true,
                     _ => false,
                 };
                 Uniformity {