Why does intpromote spew warnings for ~ operator?

[Steven Schveighoffer]

```d
import std.stdio;
import std.meta;
void main()
{
     int nbad;
     foreach(T; AliasSeq!(short, ushort))
     {
         foreach(int i; T.min .. T.max + 1)
         {
             T x = cast(T)i;
             T y = ~x;
             T newy = cast(T)~cast(int)x;
             if(y != newy)
             {
                 writefln("%s: x: %s, y: %s, newy: %s", T.stringof, x, 
y, newy);
                 ++nbad;
             }
         }
     }
     writeln("Number of affected cases: ", nbad);
}
```

Output:

```
onlineapp.d(11): Deprecation: integral promotion not done for `~x`, use 
'-preview=intpromote' switch or `~cast(int)(x)`
onlineapp.d(11): Deprecation: integral promotion not done for `~x`, use 
'-preview=intpromote' switch or `~cast(int)(x)`
Number of affected cases: 0
```

Note that with `-de` these become errors. With the -preview=intpromote 
switch, the uncasted negation compiles and runs as expected.

Right now, the deprecation is telling users *under penalty of not 
compiling* to do something that will achieve nothing when intpromote is 
enabled.

-Steve