cgen: narrow 2-byte deref stores in the wwstage assign arm

The plain `*p = v` arm's storeop map handled 1- and 4-byte pointees
but let a 2-byte one fall to MOVQ, an 8-byte over-store that clobbers
six adjacent bytes (masked in the pinned fixture by the lone frame
slot; loud with `&a[0]` of a [4]i16). cstage fldstoreop maps 2 to
MOVW. Graduates r660_i16_out_param_negative.

selfhost/cmd/wcc/cgenexpr.ww

@@ -8955,7 +8955,8 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 														typenodeprimresolved(c, pe, &ps, &uns);
 													};
 													if (ps == 1) { storeop = "MOVB"; }
-													else { if (ps == 4) { storeop = "MOVL"; }; };
+													else { if (ps == 2) { storeop = "MOVW"; }
+													else { if (ps == 4) { storeop = "MOVL"; }; }; };
 												}; }; };
 											};
 											// A slice IS the same 3-word {ptr,len,cap}