[X86][SSE] Added missing PACKSS/PACKUS intrinsic schedules

Improves atom scheduler test coverage (to make it easier to upgrade them for PR32431).

Checked on Agner that these actually match the UNPACK schedules, but better to include a separate class

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309701 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed