[x86] Make the v8i16 clever single-input shuffle lowering usable for

[x86] Make the v8i16 clever single-input shuffle lowering usable for
repeated 128-bit lane shuffles of wider vector types and use it to lower
256-bit v16i16 vector shuffles where applicable.

This should let us perfectly lowering the pattern of pshuflw and pshufhw
even for AVX2 256-bit patterns.

I've not added AVX-512 support, but it should be trivial for someone
working on that to wire up.

Note that currently this generates bad, long shuffle chains because we
don't combine 256-bit target shuffles. The subsequent patches will fix
that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230751 91177308-0d34-0410-b5e6-96231b3b80d8