[x86] Initial improvements to the new shuffle lowering for v16i8

shuffles specifically for cases where a small subset of the elements in
the input vector are actually used.

This is specifically targetted at improving the shuffles generated for
trunc operations, but also helps out splat-like operations.

There is still some really low-hanging fruit here that I want to address
but this is a huge step in the right direction.

llvm-svn: 212680