[Complexity] Optimize imult1616()
# Bug description
The function `imult1616()` is currently using two operators for the 16x16 integer operation. However, the BASOP operator `i_mult(v1, v2)` could be used instead.
```
Word16 imult1616( Word16 x, Word16 y )
{
assert( (int) x * (int) y < 32768 && (int) x * (int) y >= -32768 );
#ifdef PATCH
return i_mult( x, y );
#else
return extract_l( L_mult0( x, y ) );
#endif
}
```
issue