[llvm] [ADT] Add implementations for mulhs and mulhu to APInt (PR #84609)

[Shourya Goel via llvm-commits]

Sh0g0-1758 wrote:

@RKSimon, reverting the assertion statements because `DivisionByConstantTest` won't pass if it is left implemented, since it has 

```
template <typename Fn> static void EnumerateAPInts(unsigned Bits, Fn TestFn) {
  APInt N(Bits, 0);
  do {
    TestFn(N);
  } while (++N != 0);
}
```

Which means the number of bits of the two APInts can't be same. Please correct me if I am wrong. 

https://github.com/llvm/llvm-project/pull/84609