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

[David Green via llvm-commits]

davemgreen wrote:

Can you update the description in github with the one in https://github.com/llvm/llvm-project/pull/99418/commits/8d81896a8b1f1137a2a182597551047c1092b1da (or the others if you want to combine them)? LLVM uses a squash-and-merge approach to committing PR's, so they will be squashed into a single commit.

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