Improved flow analysis performance for variable initialization.

These changes improve **kotlinc** performance for [our project](https://github.com/LWJGL/lwjgl3) with a 15% reduction in compilation time (from 41 sec down to 35 sec).

The majority of the improvement comes from the first commit and the optimization of mergeIncomingEdgesDataForInitializers. It was at the very top in profiling runs for our code and dropped to 4th place after these changes.