also test simd_select_bitmask on arrays for less than 8 elements

src/tools/miri/tests/pass/portable-simd.rs

@@ -266,7 +266,12 @@ fn simd_mask() {
         }
     }
 
-    // This used to cause an ICE.
+    // This used to cause an ICE. It exercises simd_select_bitmask with an array as input.
+    let bitmask = u8x4::from_array([0b00001101, 0, 0, 0]);
+    assert_eq!(
+        mask32x4::from_bitmask_vector(bitmask),
+        mask32x4::from_array([true, false, true, true]),
+    );
     let bitmask = u8x8::from_array([0b01000101, 0, 0, 0, 0, 0, 0, 0]);
     assert_eq!(
         mask32x8::from_bitmask_vector(bitmask),