test/804: u16 row exercises the MOVW copy-tail arm (#35 review)

esz 1/4/8/24/56 left the esz=2 MOVW tail the only emitted-but-
unexercised copy arm in both stages' delete lowering. 41 -> 44
fixtures.

test/wcc/804_delete_elem.c

@@ -27,6 +27,8 @@
  *   i64_last        | [7,11,13], delete(xs[2]) (no copy,     | 38
  *                   | loop body never runs)                  |
  *   i32_narrow      | []i32 esz=4 — the MOVL copy tail       | 229
+ *   u16_narrow      | []u16 esz=2 — the MOVW copy tail (the  | 26
+ *                   | last otherwise-unexercised tail arm)   |
  *   u8_narrow       | []u8 esz=1 — the MOVB copy tail        | 28
  *   cap_unchanged   | manual {ptr,len=3,cap=8} header over a | 91
  *                   | stack backing; delete middle; cap must |
@@ -137,6 +139,18 @@ static const struct row rows[] = {
 	  "};\n",
 	  229, NULL },
 
+	{ "u16_narrow",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet xs: []u16 = [];\n"
+	  "\tappend(xs, 4u16);\n"
+	  "\tappend(xs, 9u16);\n"
+	  "\tappend(xs, 2u16);\n"
+	  "\tdelete(xs[1]);\n"
+	  "\treturn (xs[0] + xs[1]): i32 + len(xs)*10;\n"
+	  "};\n",
+	  26, NULL },
+
 	{ "u8_narrow",
 	  "package main;\n"
 	  "export fn main() i32 = {\n"