[X86] Use NSW/NUW flags on ISD::TRUNCATE nodes to improve X86 PACKSS/PACKUS lowering#123956

RKSimon merged 3 commits intollvm:mainllvm/llvm-project:mainfrom

RKSimon:dag-trunc-wrapRKSimon/llvm-project:dag-trunc-wrapCopy head branch name to clipboard