[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.

llvm-svn: 230751