Switch LiveIntervalUnion from std::set to IntervalMap.

This speeds up RegAllocBasic by 20%, not counting releaseMemory which becomes
way faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121201 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed