[InstCombine] ctpop(X) + ctpop(Y) => ctpop(X | Y) if X and Y have no common bits (PR48999)

For example:

```
int src(unsigned int a, unsigned int b)
{
    return __builtin_popcount(a << 16) + __builtin_popcount(b >> 16);
}

int tgt(unsigned int a, unsigned int b)
{
    return __builtin_popcount((a << 16)  | (b >> 16));
}
```

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D101210