[PATCH] [ARM64]Fix a bug when lowing shuffle vector to EXT instruction

[Hao Liu]

Hi Tim,

Thanks for your suggestion on using APInt and std::find_if. It is very simple and easy to be understood. I've refactored the patch.

BTW, there is a minor issue for this line
    "Imm = ExpectedElt;"
It is only true when there is UNDEF in the beginning. If the mask starts from non-undef index, it should be
    "Imm = M[0]"
For example, "shuffle V0, V1, <6, 7, 0, 1>" (V0, V1 are vectors of 4 elements), Imm should be "6" not "2".

Thanks,
-Hao

http://reviews.llvm.org/D3500

Files:
  lib/Target/ARM64/ARM64ISelLowering.cpp
  test/CodeGen/ARM64/ext.ll
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