Havn’t found this anywhere, so I had to do it myself. The title says it all, so here it is without drama:
// mm7 = 0×8000 0×8000 0×8000 0×8000
// mm3 = 0×7FFF 0xFFFF 0×7FFF 0xFFFF
// mm5 = 0×7FFF 0×7FFF 0xFFFF 0xFFFF
movq mm6, mm7
pand mm7, mm5 /* b ? 0 | 0×8000 */
pand mm6, mm3 /* a ? 0 | 0×8000 */
pxor mm5, [...]