IntervalMap iterators are heavyweight, so avoid copying them around and use

IntervalMap iterators are heavyweight, so avoid copying them around and use
references instead.

Similarly, IntervalMap::begin() is almost as expensive as find(), so use find(x)
instead of begin().advanceTo(x);

This makes RegAllocBasic run another 5% faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121344 91177308-0d34-0410-b5e6-96231b3b80d8