[X86] Correct some bad FileCheck prefixes in tests. Add test cases for v64i8 padd/psub saturation intrinsics.

For some reason we had the 128/256-bit tests, but no the 512-bit tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339840 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed