[X86] Use pack instructions for packus/ssat truncate patterns when 256-bit is...

[X86] Use pack instructions for packus/ssat truncate patterns when 256-bit is the largest legal vector and the result type is at least 256 bits.

Since the input type is larger than 256-bits we'll need to some
concatenating to reassemble the results. The pack instructions
ability to concatenate while packing make this a shorter/faster
sequence.

llvm-svn: 374643