[PATCH] [X86][SSE] Avoid shuffles with zero by using pshufb to create zeros

[Simon Pilgrim]

Hi chandlerc, spatel, andreadb,

pshufb can shuffle in zero bytes as well as bytes from a source vector - we can use this to avoid having to shuffle 2 vectors and ORing the result when the used inputs from a vector are all zeroable.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6878

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/vector-shuffle-128-v16.ll

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