[X86] Add test case for v8i64->v8i8 truncate with avx512 and prefer-vector-width/min-legal-vector-width=256. NFC
[X86] Add test case for v8i64->v8i8 truncate with avx512 and prefer-vector-width/min-legal-vector-width=256. NFC

With vpmovqb, we should be able to do better here until we get
AVX512VBMI on Cannonlake/Icelake.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@373569 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm