Add SSE2 trivial aliases and conversions.

`_mm_cvtsd_f64`, `_mm_cvtsd_si64x` and `_mm_cvttsd_si64x`.
See rust-lang#40.

src/x86/sse2.rs

@@ -1795,6 +1795,13 @@ pub unsafe fn _mm_cvtsd_si64(a: f64x2) -> i64 {
     cvtsd2si64(a)
 }
 
+/// Alias for [`_mm_cvtsd_si64`](fn._mm_cvtsd_si64_ss.html).
+#[cfg(target_arch = "x86_64")]
+#[inline(always)]
+#[target_feature = "+sse2"]
+#[cfg_attr(test, assert_instr(cvtsd2si))]
+pub unsafe fn _mm_cvtsd_si64x(a: f64x2) -> i64 { _mm_cvtsd_si64(a) }
+
 /// Convert the lower double-precision (64-bit) floating-point element in `b`
 /// to a single-precision (32-bit) floating-point element, store the result in
 /// the lower element of the return value, and copy the upper element from `a`
@@ -1806,6 +1813,14 @@ pub unsafe fn _mm_cvtsd_ss(a: f32x4, b: f64x2) -> f32x4 {
     cvtsd2ss(a, b)
 }
 
+/// Return the lower double-precision (64-bit) floating-point element of "a".
+#[inline(always)]
+#[target_feature = "+sse2"]
+// no particular instruction to test
+pub unsafe fn _mm_cvtsd_f64(a: f64x2) -> f64 {
+    a.extract(0)
+}
+
 /// Convert the lower single-precision (32-bit) floating-point element in `b`
 /// to a double-precision (64-bit) floating-point element, store the result in
 /// the lower element of the return value, and copy the upper element from `a`
@@ -1845,6 +1860,13 @@ pub unsafe fn _mm_cvttsd_si64(a: f64x2) -> i64 {
     cvttsd2si64(a)
 }
 
+/// Alias for [`_mm_cvttsd_si64`](fn._mm_cvttsd_si64_ss.html).
+#[cfg(target_arch = "x86_64")]
+#[inline(always)]
+#[target_feature = "+sse2"]
+#[cfg_attr(test, assert_instr(cvttsd2si))]
+pub unsafe fn _mm_cvttsd_si64x(a: f64x2) -> i64 { _mm_cvttsd_si64(a) }
+
 /// Convert packed single-precision (32-bit) floating-point elements in `a` to
 /// packed 32-bit integers with truncation.
 #[inline(always)]
@@ -4022,6 +4044,12 @@ mod tests {
         );
     }
 
+    #[simd_test = "sse2"]
+    unsafe fn _mm_cvtsd_f64() {
+        let r = sse2::_mm_cvtsd_f64(f64x2::new(-1.1, 2.2));
+        assert_eq!(r, -1.1);
+    }
+
     #[simd_test = "sse2"]
     unsafe fn _mm_cvtss_sd() {
         use std::{f32, f64};