[X86] Allow combineTruncateWithSat to use pack instructions for i16->i8 without AVX512BW.

[X86] Allow combineTruncateWithSat to use pack instructions for i16->i8 without AVX512BW.

We need AVX512BW to be able to truncate an i16 vector. If we don't
have that we have to extend i16->i32, then trunc, i32->i8. But we
won't be able to remove the min/max if we do that. At least not
without more special handling.

llvm-svn: 368623