[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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212680
91177308-0d34-0410-b5e6-
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