Remove the unused IntervalPartition analysis pass (llvm#88133)

This removes the old legacy PM "intervals" analysis pass (aka
IntervalPartition). It also removes the associated Interval and
IntervalIterator help classes.

Reasons for removal:
1) The pass is not used by llvm-project (not even being tested by
   any regression tests).
2) Pass has not been ported to new pass manager, which at least
   indicates that it isn't used by the middle-end.
3) ASan reports heap-use-after-free on
      ++I;  // After the first one...
   even if false is passed to intervals_begin. Not sure if that is
   a false positive, but it makes the code a bit less trustworthy.