Change remaining _undefined_ functions to zero-init

Author: saethlin

crates/core_arch/src/x86/avx.rs

@@ -2719,8 +2719,7 @@ pub unsafe fn _mm256_undefined_pd() -> __m256d {
 // This intrinsic has no corresponding instruction.
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub unsafe fn _mm256_undefined_si256() -> __m256i {
-    // FIXME: this function should return MaybeUninit<__m256i>
-    mem::MaybeUninit::<__m256i>::uninit().assume_init()
+    __m256i(0, 0, 0, 0)
 }
 
 /// Sets packed __m256 returned vector with the supplied values.

crates/core_arch/src/x86/sse2.rs

@@ -2736,8 +2736,7 @@ pub unsafe fn _mm_castsi128_ps(a: __m128i) -> __m128 {
 #[target_feature(enable = "sse2")]
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub unsafe fn _mm_undefined_pd() -> __m128d {
-    // FIXME: this function should return MaybeUninit<__m128d>
-    mem::MaybeUninit::<__m128d>::uninit().assume_init()
+    __m128d(0.0, 0.0)
 }
 
 /// Returns vector of type __m128i with undefined elements.
@@ -2747,8 +2746,7 @@ pub unsafe fn _mm_undefined_pd() -> __m128d {
 #[target_feature(enable = "sse2")]
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub unsafe fn _mm_undefined_si128() -> __m128i {
-    // FIXME: this function should return MaybeUninit<__m128i>
-    mem::MaybeUninit::<__m128i>::uninit().assume_init()
+    __m128i(0, 0)
 }
 
 /// The resulting `__m128d` element is composed by the low-order values of