[PERF] Get rid of MultiIndex conversion in IntervalIndex.is_monotonic methods
- xref #24813
-  passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


```
All benchmarks:

       before           after         ratio
     [85572de5]       [445fbe56]
     <master>         <GH24813-inc-dec>
-         261±6ns          134±3ns     0.51  index_object.IntervalIndexMethod.time_monotonic_inc

```