[LoopSimplifyCFG] Use DTU.applyUpdates instead of insert/deleteEdge

`insert/deleteEdge` methods in DTU can make updates incorrectly in some cases
(see https://bugs.llvm.org/show_bug.cgi?id=40528), and it is recommended to
use `applyUpdates` methods instead when it is needed to make a mass update in CFG.

Differential Revision: https://reviews.llvm.org/D57316
Reviewed By: kuhar

llvm-svn: 353502