Add SIMD vector type tests

src/test/compile-fail/simd-type.rs

+#[simd]
+struct vec4<T>(T, T, T, T); //~ ERROR SIMD vector cannot be generic
+
+#[simd]
+struct empty; //~ ERROR SIMD vector cannot be empty
+
+#[simd]
+struct i64f64(i64, f64); //~ ERROR SIMD vector should be homogeneous
+
+#[simd]
+struct int4(int, int, int, int); //~ ERROR SIMD vector element type should be machine type
+
+fn main() {}

src/test/run-pass/simd-type.rs

+#[simd]
+struct RGBA {
+    r: f32,
+    g: f32,
+    b: f32,
+    a: f32
+}
+
+fn main() {}