[SimplifyCFG] If provided, preserve Dominator Tree

SimplifyCFG is an utility pass, and the fact that it does not
preserve DomTree's, forces it's users to somehow workaround that,
likely by not preserving DomTrees's themselves.

Indeed, simplifycfg pass didn't know how to preserve dominator tree,
it took me just under a month (starting with e1133179)
do rectify that, now it fully knows how to,
there's likely some problems with that still,
but i've dealt with everything i can spot so far.

I think we now can flip the switch.

Note that this is functionally an NFC change,
since this doesn't change the users to pass in the DomTree,
that is a separate question.

Reviewed By: kuhar, nikic

Differential Revision: https://reviews.llvm.org/D94827