[llvm] [DAGCombiner] Don't let scalarizeBinOpOfSplats create illegal scalar MULHS/MULHU (PR #104518)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Are there other ops that can't be type legalized? If other cases arise then we should maybe revert and revisit #100749

https://github.com/llvm/llvm-project/pull/104518