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

[Jay Foad via llvm-commits]

https://github.com/jayfoad requested changes to this pull request.

Please remove the "lo" functions as they are identical to `operator*`. For the "hi" functions I'd prefer the names mulhu and mulhs, since these names are already used elsewhere in LLVM (in KnownBits and in SelectionDAG).

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