[CodeGen][SimplifyCFG] Teach DwarfEHPrepare to preserve DomTree

Once the default for SimplifyCFG flips, we can no longer pass nullptr
instead of DomTree to SimplifyCFG, so we need to propagate it here.

We don't strictly need to actually preserve DomTree in DwarfEHPrepare,
but we might as well do it, since it's trivial.