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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Yeah the improvements look good, it's good to see it handles more types now. If we can get the TRUNCATE_SSAT_U nodes working then this sounds OK. It should match from either `smax(smin(x, 255), 0)` or `smax(umin(x, 255), 0)`

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