[CodeGen][DwarfEHPrepare] Preserve Dominator Tree

Now that D94827 has flipped the switch, and SimplifyCFG is officially marked
as production-ready regarding Dominator Tree preservation,
we can update this user pass to also preserve Dominator Tree.

This is a geomean compile-time win of `-0.05%`..`-0.08%`.
https://llvm-compile-time-tracker.com/compare.php?from=51a25846c198cff00abad0936f975167357afa6f&to=082499aac236a5c141e50a9e77870d5be2de5f0b&stat=instructions

Differential Revision: https://reviews.llvm.org/D95548