[X86] Add test case for v16i64->v16i32 truncate on min-legal-vector-width=256.
[X86] Add test case for v16i64->v16i32 truncate on min-legal-vector-width=256.

I think this case would crash before I added back the -x86-experimental-vector-widening command line option. Adding this test case to prevent breaking it again when we remove the option.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371673 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm