Resolve old FIXME comments (rust-lang#1364)

Author: Nugine

crates/core_arch/src/wasm32/simd128.rs

@@ -2349,10 +2349,6 @@ pub use i8x16_all_true as u8x16_all_true;
 #[doc(alias("i8x16.bitmask"))]
 #[stable(feature = "wasm_simd", since = "1.54.0")]
 pub fn i8x16_bitmask(a: v128) -> u16 {
-    // FIXME(https://bugs.llvm.org/show_bug.cgi?id=50507) - this produces an
-    // extraneous `i32.and` instruction against a mask of 65535 when converting
-    // from the native intrinsic's i32 return value to our desired u16. This
-    // shouldn't be necessary, though, but requires upstream LLVM changes.
     unsafe { llvm_bitmask_i8x16(a.as_i8x16()) as u16 }
 }
 
@@ -3460,7 +3456,7 @@ pub use i32x4_extmul_high_u16x8 as u32x4_extmul_high_u16x8;
 
 /// Lane-wise wrapping absolute value.
 #[inline]
-// #[cfg_attr(test, assert_instr(i64x2.abs))] // FIXME llvm
+#[cfg_attr(test, assert_instr(i64x2.abs))]
 #[target_feature(enable = "simd128")]
 #[doc(alias("i64x2.abs"))]
 #[stable(feature = "wasm_simd", since = "1.54.0")]

crates/core_arch/src/x86/avx2.rs

@@ -527,7 +527,8 @@ pub unsafe fn _mm256_broadcastd_epi32(a: __m128i) -> __m256i {
 /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm_broadcastq_epi64)
 #[inline]
 #[target_feature(enable = "avx2")]
-// FIXME: https://github.com/rust-lang/stdarch/issues/791
+// Emits `vmovddup` instead of `vpbroadcastq`
+// See https://github.com/rust-lang/stdarch/issues/791
 #[cfg_attr(test, assert_instr(vmovddup))]
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub unsafe fn _mm_broadcastq_epi64(a: __m128i) -> __m128i {