Encode [u|s]widen_low for x86

cranelift/filetests/filetests/isa/x86/simd-conversion-binemit.clif

@@ -1,6 +1,6 @@
 test binemit
 set enable_simd
-target x86_64 has_ssse3=true
+target x86_64 nehalem
 
 ; Ensure raw_bitcast emits no instructions.
 function %raw_bitcast_i16x8_to_b32x4() {
@@ -17,3 +17,10 @@ block0(v0: i32x4 [%xmm6], v1: i32x4 [%xmm4]):
 [-, %xmm6]  v3 = x86_palignr v0, v1, 3      ; bin: 66 0f 3a 0f f4 03
             return
 }
+
+function %conversions_i16x8(i16x8) {
+block0(v0: i16x8 [%xmm6]):
+[-, %xmm2]  v1 = swiden_low v0              ; bin: 66 0f 38 23 d6
+[-, %xmm11] v2 = uwiden_low v0              ; bin: 66 44 0f 38 33 de
+            return
+}