[PATCH] Generate better code for shuffles

[Michael Kuperstein]

Hi nadav, andreadb, spatel, chandlerc, delena,

This fixes PR15872, the code improves from:

```
        vpextrd $1, %xmm0, %eax
        vmovd   %xmm0, %ecx
        vmovd   %ecx, %xmm1
        vpinsrd $1, %eax, %xmm1, %xmm1
        vextractf128    $1, %ymm0, %xmm2
        vmovd   %xmm2, %eax
        vpinsrd $2, %eax, %xmm1, %xmm1
        vpextrd $1, %xmm2, %eax
        vpinsrd $3, %eax, %xmm1, %xmm1
        vpextrd $3, %xmm0, %eax
        vpextrd $2, %xmm0, %ecx
        vmovd   %ecx, %xmm0
        vpinsrd $1, %eax, %xmm0, %xmm0
        vpextrd $2, %xmm2, %eax
        vpinsrd $2, %eax, %xmm0, %xmm0
        vpextrd $3, %xmm2, %eax
        vpinsrd $3, %eax, %xmm0, %xmm0
        vmovdqa %xmm1, (%rdi)
        vzeroupper
        retq
```

to

```
        vextractf128    $1, %ymm0, %xmm1
        vpunpcklqdq     %xmm1, %xmm0, %xmm2 # xmm2 = xmm0[0],xmm1[0]
        vpunpckhqdq     %xmm1, %xmm0, %xmm0 # xmm0 = xmm0[1],xmm1[1]
        vmovdqa %xmm2, (%rdi)
        vzeroupper
        retq
```

Sanjay has a fix for PR21711 which apparently has the same underlying issue here: http://reviews.llvm.org/D6622

This version is more general, but it may be too general, I'm fine with anything in this vein that fixes both PRs.

http://reviews.llvm.org/D6678

Files:
  include/llvm/Target/TargetLowering.h
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/vector-shuffle-combining.ll

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