Fix transmuting from vector type to ScalarPair type

Fixes rust-lang#1292

Author: bjorn3

example/std_example.rs

@@ -164,6 +164,8 @@ unsafe fn test_simd() {
     let cmp_eq = _mm_cmpeq_epi8(y, y);
     let cmp_lt = _mm_cmplt_epi8(y, y);
 
+    let (zero0, zero1) = std::mem::transmute::<_, (u64, u64)>(x);
+    assert_eq!((zero0, zero1), (0, 0));
     assert_eq!(std::mem::transmute::<_, [u16; 8]>(or), [7, 7, 7, 7, 7, 7, 7, 7]);
     assert_eq!(std::mem::transmute::<_, [u16; 8]>(cmp_eq), [0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff]);
     assert_eq!(std::mem::transmute::<_, [u16; 8]>(cmp_lt), [0, 0, 0, 0, 0, 0, 0, 0]);

src/value_and_place.rs

@@ -590,7 +590,10 @@ impl<'tcx> CPlace<'tcx> {
                 return;
             }
             CPlaceInner::VarPair(_local, var1, var2) => {
-                let (data1, data2) = CValue(from.0, dst_layout).load_scalar_pair(fx);
+                let (ptr, meta) = from.force_stack(fx);
+                assert!(meta.is_none());
+                let (data1, data2) =
+                    CValue(CValueInner::ByRef(ptr, None), dst_layout).load_scalar_pair(fx);
                 let (dst_ty1, dst_ty2) = fx.clif_pair_type(self.layout().ty).unwrap();
                 transmute_value(fx, var1, data1, dst_ty1);
                 transmute_value(fx, var2, data2, dst_ty2);