[SimplifyCFG][AMDGPU] AMDGPUUnifyDivergentExitNodes: SimplifyCFG isn't ready...

[SimplifyCFG][AMDGPU] AMDGPUUnifyDivergentExitNodes: SimplifyCFG isn't ready to preserve PostDomTree

There is a number of transforms in SimplifyCFG that take DomTree out of
DomTreeUpdater, and do updates manually. Until they are fixed,
user passes are unable to claim that PDT is preserved.

Note that the default for SimplifyCFG is still not to preserve DomTree,
so this is still effectively NFC.