[X86] Test SKX cpu in the vector-trunc-packus/ssat/usat.ll tests instad of min-legal...

[X86] Test SKX cpu in the vector-trunc-packus/ssat/usat.ll tests instad of min-legal-vector-width.ll

This adds "min-legal-vector-width"="256" function attributes to
all the tests for a larger than 256-bit input. Also switch any
larger than 512-bit inputs to use a load. This makes the
arguments consistent with min-legal-vector-width attribute which
should usually be at least as large as the arguments.

The SKX configuration will avoid using zmm registers on the
modified test cases. For many of them we should use something
closer to the AVX2 codegen with pack instructions instead of
the avx512 saturating truncates.

llvm-svn: 374642