[NFCI][CodeGen] DwarfEHPrepare: don't actually pass DTU into simplifyCFG by default

also, don't verify DomTree unless we intend to maintain it.
This is a very dumb think-o, i guess i was even warned about it
by subconsciousness in 4b806473's
commit message..

Fixes a compile-time regression reported by Martin Storsjö
in post-commit review of 2461cdb4.