[llvm] [DAG] Use known-bits when creating umulh/smulh. (PR #160916)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@davemgreen If you regenerate combine-pmuldq.ll you should be good to go - I'll continue https://github.com/llvm/llvm-project/pull/188480 as a follow up once this has landed

https://github.com/llvm/llvm-project/pull/160916