Subzero: Translation-time improvements in the register allocator.

1. Use a sorted std::vector instead of std::set to improve management
of the Unhandled sets.  This is the main performance gain.

2. Use std::list.splice() to move items between lists, instead of
erase()+push_back().  This doesn't really save much, but the intention
is somewhat clearer.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/642603005