[PATCH] [x86] Teach the X86 backend to DAG-combine SSE2 shuffles that are trivially redundant.

[Chandler Carruth]

Hi grosbach, filcab,

This fixes several cases in the new vector shuffle lowering algorithm which would generate redundant shuffle instructions for the sake of simplicity.

I'm also deleting a testcase which was somewhat ridiculous. It was checking for a bug in 2007 about incorrectly transforming shuffles by looking for the string "-86" in the output of a pretty substantial function. This test case doesn't seem to have any value at this point.

Builds on top of: http://reviews.llvm.org/D4225

http://reviews.llvm.org/D4240

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/2007-09-18-ShuffleXformBug.ll
  test/CodeGen/X86/vector-shuffle-128-v8.ll
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