[llvm] [ADT] Add signed and unsigned mulHi and mulLo to APInt (PR #84719)

[Jay Foad via llvm-commits]

================
@@ -2193,6 +2193,12 @@ inline const APInt absdiff(const APInt &A, const APInt &B) {
   return A.uge(B) ? (A - B) : (B - A);
 }
 
+/// Return the high bits of the signed multiplication of C1 and C2
----------------
jayfoad wrote:

Uber-nit: comments should be full sentences, per coding standards.
```suggestion
/// Return the high bits of the signed multiplication of C1 and C2.
```

https://github.com/llvm/llvm-project/pull/84719