[PATCH] [X86][SSE] Improve legal SHUFP and PSHUFD shuffle matching

[Simon Pilgrim]

Hi andreadb, chandlerc,

Updated X86TargetLowering::isShuffleMaskLegal to match SHUFP masks with commuted inputs and PSHUFD masks that reference the second input.

As part of this I've refactored isPSHUFDMask to work in a more general manner and allow it to match against either the first or second input vector.

As discussed with Andrea, further changes should probably wait until the old shuffle lowering code is dropped, but these initial changes were beneficial and easy to isolate.

http://reviews.llvm.org/D6287

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/vector-shuffle-256-v4.ll
  test/CodeGen/X86/vector-shuffle-combining.ll
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