[Attributor] Make use of analysis in the MustBeExecutedExplorer

[jdoerfert]

Right now the MustBeExecutedContextExplorer Explorer is created without analysis, e.g., without a dominator tree. As a consequence it is not able to determine non-trivial control flow merge points which reduces the "must-be-executed-context" substantially.

The task is to provide the appropriate analysis getters to the Explorer during creation and update the tests accordingly. If tests for the new capabilities are missing, thus the affected tests do not show the expected effect, new tests need to be added.

[baziotis]

It's great that you made these issues. :) I think it would be good to refer to them somehow from the GSoC projects page.