[X86] Enable use of avx512 saturating truncate instructions in more cases.

This enables use of the saturating truncate instructions when the
result type is less than 128 bits. It also enables the use of
saturating truncate instructions on KNL when the input is less
than 512 bits. We can do this by widening the input and then
extracting the result.

llvm-svn: 374731