This patch teaches the DAGCombiner how to fold insert_subvector nodes
when the input is a concat_vectors and the insert replaces one of the
concat halves:

Lower half: fold (insert_subvector (concat_vectors X, Y), Z) ->
(concat_vectors Z, Y)
Upper half: fold (insert_subvector (concat_vectors X, Y), Z) ->
(concat_vectors X, Z)

This can be seen with the following IR:

define <8 x float> @lower_half(<4 x float> %v1, <4 x float> %v2, <4 x
float> %v3) {
  %1 = shufflevector <4 x float> %v1, <4 x float> %v2, <8 x i32> <i32
0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
  %2 = tail call <8 x float> @llvm.x86.avx.vinsertf128.ps.256(<8 x
float> %1, <4 x float> %v3, i8 0)

The vinsertf128 intrinsic is converted into an insert_subvector node
in SelectionDAGBuilder.cpp.

Using AVX, without the patch this generates two vinsertf128 instructions:

vinsertf128 $1, %xmm1, %ymm0, %ymm0
vinsertf128 $0, %xmm2, %ymm0, %ymm0

With the patch this is optimized into:

vinsertf128 $1, %xmm1, %ymm2, %ymm0

Patch by Robert Lougher.