[Dominators] Make applyUpdate's documentation less confusing [NFC]

Summary:
It was pointed out by @chandlerc that it's not clear whether both applyUpdates and insert/deleteEdge can be used to perform multiple updates.

IMO, the confusing part was that the comment above applyUpdates made a comparison of expected update time between calling it and calling insert/deleteEdge multiple times. It's generally not possible to safely call insert/deleteEdge multiple times, which documentation for each of the 3 functions warns about, so the whole comparison makes very little sense. On top of that, the comment is already lengthy, so I think it's best to just get rid of this comparison.

Reviewers: chandlerc, asbirlea, NutshellySima, grosser

Reviewed By: chandlerc

Subscribers: llvm-commits, chandlerc

Differential Revision: https://reviews.llvm.org/D49944

llvm-svn: 338184