[PATCH] [X86][SSE] pslldq/psrldq byte shifts/rotation for SSE2

[Simon Pilgrim]

Hi chandlerc, andreadb, spatel,

This patch builds on http://reviews.llvm.org/D5598 to improve byte shift/rotation shuffles on pre-SSSE3 (palignr) targets.

I've also made use of the ability of the SLLDQ/SRLDQ instructions to implicitly shift in zero bytes to avoid the need to create a zero register if we had used palignr with a zero register.

http://reviews.llvm.org/D5699

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/palignr.ll
  test/CodeGen/X86/vector-shuffle-128-v16.ll
  test/CodeGen/X86/vector-shuffle-128-v8.ll
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