Improve dominator tree building & walking (dotnet/coreclr#27282)

Improve dominator tree building & walking (dotnet/coreclr#27282)

* Delete unused SsaBuilder code

* Delete BasicBlock::bbDfsNum

bbPostOrderNum serves the same purpose

* Improve dominator tree building & walking
- Change the dominator tree data structure to a simple array of nodes
- Adapt fgBuildDomTree so it can be used by SsaBuilder
- Add a dominator tree visitor class
- Reuse the dominator tree built by SsaBuilder for copy propagation

* Move bbNatLoopNum to fill a padding hole

Commit migrated from https://github.com/dotnet/coreclr/commit/9251ceff0395f989253b7874862feb396b7bddd3