[DAGCombiner] narrow shuffle of concatenated vectors

// shuffle (concat X, undef), (concat Y, undef), Mask -->
// concat (shuffle X, Y, Mask0), (shuffle X, Y, Mask1)

The ARM changes with 'vtrn' and narrowed 'vuzp' are improvements.

The x86 changes look neutral or better. There's one test with an
extra instruction, but that could be reversed for a subtarget with
the right attributes. But by default, we want to avoid the 256-bit
op when possible (in my motivating benchmark, a handful of ymm ops
sprinkled into a sequence of xmm ops are triggering frequency
throttling on Haswell resulting in significantly worse perf).

Differential Revision: https://reviews.llvm.org/D60545

llvm-svn: 358291