Be a bit more efficient when processing the active and inactive

Be a bit more efficient when processing the active and inactive
lists. Instead of scanning the vector backwards, scan it forward and
swap each element we want to erase. Then at the end erase all removed
intervals at once. This doesn't save much: 0.08s out of 4s when
compiling 176.gcc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16136 91177308-0d34-0410-b5e6-96231b3b80d8