[x86] Add handling for splat-like widenings of v16i8 shuffles.

These show up really frequently, not the least with actual splats. =] We
lowered these quite badly before. The new code path tries to widen i8
shuffles to i16 shuffles in a splat-like way. There are still some
inefficiencies in our i16 splat logic though, so we aren't really done
here.

Also, for certain patterns (bit of a gather-and-splat) we still
generate pretty silly code, and I've left a fixme for addressing it.
However, I'm not actually worried about this code pattern as much. The
old shuffle lowering generates a 29 instruction monstrosity for it that
should execute much more slowly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211974 91177308-0d34-0410-b5e6-96231b3b80d8
Fix this test to not write to the source tree, and instead to write to
a temporary file. This fixes the test in cases where the source tree is
mounted read-only.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211975 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Fix a bug in the v8i16 shuffling exposed by the new splat-like
lowering for v16i8.

ASan and some bots caught this bug with existing test cases. Fixing it
even fixed a miscompile with one of the test cases. I'm still a bit
suspicious of this test case as I've not taken a proper amount of time
to think about it, but the fix here is strict goodness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211976 91177308-0d34-0410-b5e6-96231b3b80d8