[X86] Update trunc_packus_v32i32_v32i8 test in min-legal-vector-width.ll to...

[X86] Update trunc_packus_v32i32_v32i8 test in min-legal-vector-width.ll to use a load for the large type and add the min-legal-vector-width attribute.

The attribute is needed to avoid zmm registers. Using memory
avoids argument splitting for large vectors.

llvm-svn: 374486