[llvm] [DAG] Support saturated truncate (PR #99418)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> @RKSimon @davemgreen Is it okay to reduce commits?

Sure - as I mentioned above the commit summary at the top needs editing. Once that's done I'll approve.

https://github.com/llvm/llvm-project/pull/99418