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

[Shourya Goel via llvm-commits]

Sh0g0-1758 wrote:

@jayfoad, regarding changes in `KnownBitsTest.cpp`, is it expected to change `return KnownBits::mulhs(Known1, Known2);` with `return APIntOps::mulhs(Known1, Known2)`? Since KnownBits have its own implementation of mulhs and mulhu, I am bit confused if that is the right thing to do. 

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