UseListOrder: Use std::vector

I initially used a `SmallVector<>` for `UseListOrder::Shuffle`, which
was a silly choice.  When I realized my error I quickly rolled a custom
data structure.

This commit simplifies it to a `std::vector<>`.  Now that I've had a
chance to measure performance, this data structure isn't part of a
bottleneck, so the additional complexity is unnecessary.

This is part of PR5680.

llvm-svn: 214979