DAGCombiner: Fold a shuffle on CONCAT_VECTORS into a new CONCAT_VECTORS if possible.

This pattern occurs in SROA output due to the way vector arguments are lowered
on ARM.

The testcase from PR15525 now compiles into this, which is better than the code
we got with the old scalarrepl:
_Store:
	ldr.w	r9, [sp]
	vmov	d17, r3, r9
	vmov	d16, r1, r2
	vst1.8	{d16, d17}, [r0]
	bx	lr

Differential Revision: http://llvm-reviews.chandlerc.com/D647