IR: Optimize size of use-list order shuffle vectors

Since we're storing lots of these, save two-pointers per vector with a
custom type rather than using the relatively heavy `SmallVector`.

Part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214135 91177308-0d34-0410-b5e6-96231b3b80d8