Add test for compound assignment with swizzle (#4581)

Author: sudonatalie

src/webgpu/shader/validation/decl/assignment_statement.spec.ts

@@ -80,3 +80,17 @@ g.test('vector_swizzle_assignment')
       }`;
     t.expectCompileResult(kSwizzleTests[t.params.case].pass, wgsl);
   });
+
+g.test('compound_assignment_with_swizzle')
+  .desc('Test compound assignment of a vector with a swizzle on the rhs.')
+  .fn(t => {
+    const code = `
+      @fragment
+      fn main() {
+        var v: vec3<i32> = vec3(1, 2, 3);
+        var w: vec4<i32> = vec4(10);
+        v *= w.xyz;
+      }
+    `;
+    t.expectCompileResult(true, code);
+  });