Enable simd insert and extract tests

Working since rust-lang/stdarch#876

Fixes #666

Author: bjorn3

example/std_example.rs

@@ -144,9 +144,8 @@ unsafe fn test_simd() {
     test_mm_cvtepi8_epi16();
     test_mm_cvtsi128_si64();
 
-    // FIXME(#666) implement `#[rustc_arg_required_const(..)]` support
-    //test_mm_extract_epi8();
-    //test_mm_insert_epi16();
+    test_mm_extract_epi8();
+    test_mm_insert_epi16();
 
     let mask1 = _mm_movemask_epi8(dbg!(_mm_setr_epi8(255u8 as i8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)));
     assert_eq!(mask1, 1);