[llvm] [SDAG] Improve `SimplifyDemandedBits` for mul (PR #90034)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

The APInt::clearHighBits part LGTM if you want to pull it out into its own PR and I'll accept it

https://github.com/llvm/llvm-project/pull/90034