Changes

[SimplifyCFGPass] Ensure that DominatorTreeWrapperPass is init'd before SimplifyCFG

It's probably better than hoping that it will happen to be
already initialized.

[SimplifyCFG] Teach tryWidenCondBranchToCondBranch() to preserve DomTree

[SimplifyCFG] Teach eliminateDeadSwitchCases() to preserve DomTree, part 2

[IR] PassManagerTest: Register DominatorTreeAnalysis before running SimplifyCFGPass

Otherwise these particular tests fail when SimplifyCFG requires DomTree

[Utils] LocalTest: fix SimplifyCFGWithNullAC test to work with `-simplifycfg-require-and-preserve-domtree=1`

[NFC] clang-format the entire DwarfEHPrepare.cpp

[NFC][CodeGen] Split DwarfEHPrepare pass into an actual transform and an legacy-PM wrapper

This is consistent with the layout of other passes,
and simplifies further refinements regarding DomTree handling.

This is indended to be a NFC commit.

[NFC][CodeGen][Tests] Mark all tests that fail to preserve DomTree for SimplifyCFG as such

These tests start to fail when the SimplifyCFG's default regarding DomTree
updating is switched on, so mark them as needing changes.

[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.

[SimplifyCFG] Teach removeUndefIntroducingPredecessor to preserve DomTree

[AMDGPU][SimplifyCFG] Teach AMDGPUUnifyDivergentExitNodes to preserve {,Post}DomTree

This is a (last big?) part of the patch series to make SimplifyCFG
preserve DomTree. Currently, it still does not actually preserve it,
even thought it is pretty much fully updated to preserve it.

Once the default is flipped, a valid DomTree must be passed into
simplifyCFG, which means that whatever pass calls simplifyCFG,
should also be smart about DomTree's.

As far as i can see from `check-llvm` with default flipped,
this is the last LLVM test batch (other than bugpoint tests)
that needed fixes to not break with default flipped.

The changes here are boringly identical to the ones i did
over 42+ times/commits recently already,
so while AMDGPU is outside of my normal ecosystem,
i'm going to go for post-commit review here,
like in all the other 42+ changes.

Note that while the pass is taught to preserve {,Post}DomTree,
it still doesn't do that by default, because simplifycfg
still doesn't do that by default, and flipping default
in this pass will implicitly flip the default for simplifycfg.
That will happen, but not right now.