[DagCombine] Improve DAGCombiner BUILD_VECTOR when it has two sources of elements
This partially fixes PR21943.
For AVX, we go from:
vmovq (%rsi), %xmm0
vmovq (%rdi), %xmm1
vpermilps $-27, %xmm1, %xmm2 ## xmm2 = xmm1[1,1,2,3]
vinsertps $16, %xmm2, %xmm1, %xmm1 ## xmm1 = xmm1[0],xmm2[0],xmm1[2,3]
vinsertps $32, %xmm0, %xmm1, %xmm1 ## xmm1 = xmm1[0,1],xmm0[0],xmm1[3]
vpermilps $-27, %xmm0, %xmm0 ## xmm0 = xmm0[1,1,2,3]
vinsertps $48, %xmm0, %xmm1, %xmm0 ## xmm0 = xmm1[0,1,2],xmm0[0]
To the expected:
vmovq (%rdi), %xmm0
vmovhpd (%rsi), %xmm0, %xmm0
retq
Fixing this for AVX2 is still open.
Differential Revision: http://reviews.llvm.org/D6749
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224759
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