[AMDGPU] Preserve MachineDominatorTree in SILowerControlFlow

Updating the MachineDominatorTree is easy since SILowerControlFlow only
splits and removes basic blocks. This should save a bit of compile time
because previously we would recompute the dominator tree from scratch
after this pass.

Another reason for doing this is that SILowerControlFlow preserves
LiveIntervals which transitively requires MachineDominatorTree. I think
that means that SILowerControlFlow is obliged to preserve
MachineDominatorTree too as explained here:
https://lists.llvm.org/pipermail/llvm-dev/2020-November/146923.html
although it does not seem to have caused any problems in practice yet.

Differential Revision: https://reviews.llvm.org/D111313