[X86][SSE] Add further test cases showing failure to compute sign bits through PACKSS

Suggested in D37680

Note: had to drop AVX512VL tests as there is an infinite loop in the new tests that needs further investigation (not relevant to D37680).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312910 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed