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

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.

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

Differential Revision: http://reviews.llvm.org/D8563

llvm-svn: 233100