[X86][SSE] Add support for PACKSS nodes to faux shuffle extraction

If the inputs won't saturate during packing then we can treat the PACKSS as a truncation shuffle

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305091 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed