[PATCH] [X86, AVX] recognize shufflevector with zero input as a vperm2 (PR22984)

[Sanjay Patel]

Hi mkuper, andreadb, RKSimon, chandlerc,

vperm2x128 instructions have the special ability (aka free hardware capability) to shuffle zero values into a vector. 

This patch recognizes that type of shuffle and generates the appropriate control byte.

Note: I have a follow-on patch to convert vperm2 intrinsics with zero masks into generic shuffles. That should close the loop on this special-purpose x86 permute.

https://llvm.org/bugs/show_bug.cgi?id=22984

http://reviews.llvm.org/D8563

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/avx-vperm2x128.ll

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