[x86] Dramatically improve v8i16 shuffle lowering by not using its

terribly complex partial blend logic.

This code path was one of the more complex and bug prone when it first
went in and it hasn't faired much better. Ultimately, with the simpler
basis for unpack lowering and support bit-math blending, this is
completely obsolete. In the worst case without this we generate
different but equivalent instructions. However, in many cases we
generate much better code. This is especially true when blends or pshufb
is available.

This does expose one (minor) weakness of the unpack lowering that I'll
try to address.

In case you were wondering, this is actually a big part of what I've
been trying to pull off in the recent string of commits.

llvm-svn: 229853