[PATCH] [DAGCombine] Don't generate atrocious shuffles for constant vectors

[Michael Kuperstein]

Hi spatel, chandlerc,

According to PR22276, generating constant vectors (and, especially, zero vectors) can sometimes result in very odd code. 
This is especially true for zero vectors, where a single xor should be sufficient, but instead shuffles are generated.

This adds a DAGCombine that removes a redundant shuffle for this case. 
Note that lowering the resulting BUILD_VECTOR to a constant broadcast vs. a constant pool load vs. something more efficient (e.g. a xor for the case of 0) is a target decision, and isn't made here.

http://reviews.llvm.org/D7093

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/splat-const.ll
  test/CodeGen/X86/sse41.ll
  test/CodeGen/X86/widen_shuffle-1.ll

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