Prepare for new pass manager version of `MachineDominatorTreeAnalysis`.
We may need a machine dominator tree version of `DomTreeUpdater` to
handle `SplitCriticalEdge` in some CodeGen passes.
This reverts commit 7f230feeeac8a67b335f52bd2e900a05c6098f20.
Breaks CodeGenCUDA/link-device-bitcode.cu in check-clang,
and many LLVM tests, see comments on https://reviews.llvm.org/D121169
Previously the name of the class (AMDGPUCFGStructurizer) did not
match the name of the file (AMDILCFGStructurizer).
Standardize on the name R600MachineCFGStructurizer by analogy with
AMDGPUMachineCFGStructurizer.
Differential Revision: https://reviews.llvm.org/D120128