IR: Create the use-list order shuffle vector in-place

Per David Blaikie's review of r214135, this is a more natural way to
initialize.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214184 91177308-0d34-0410-b5e6-96231b3b80d8
Add a test for the mtriple plugin option.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214186 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-uselistorder: Fix header comments from r214144

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214187 91177308-0d34-0410-b5e6-96231b3b80d8