optimize the AVX2 (integer) version of vperm2 into a shuffle

optimize the AVX2 (integer) version of vperm2 into a shuffle

...because this is what happens when an instruction
set puts its underwear on after its pants.

This is an extension of r232852, r233100, and 233110:
http://llvm.org/viewvc/llvm-project?view=revision&revision=232852
http://llvm.org/viewvc/llvm-project?view=revision&revision=233100
http://llvm.org/viewvc/llvm-project?view=revision&revision=233110

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233127 91177308-0d34-0410-b5e6-96231b3b80d8