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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214979
91177308-0d34-0410-b5e6-
96231b3b80d8