Reuse the DominatorTree postorder travesal in BlockLoweringOrder (#5843)

* Rework the blockorder module to reuse the dom tree's cfg postorder

* Update domtree tests

* Treat br_table with an empty jump table as multiple block exits

* Bless tests

* Change branch_idx to succ_idx and fix the comment

cranelift/codegen/src/isa/mod.rs

@@ -43,6 +43,7 @@
 //! The configured target ISA trait object is a `Box<TargetIsa>` which can be used for multiple
 //! concurrent function compilations.
 
+use crate::dominator_tree::DominatorTree;
 pub use crate::isa::call_conv::CallConv;
 
 use crate::flowgraph;
@@ -252,6 +253,7 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     fn compile_function(
         &self,
         func: &Function,
+        domtree: &DominatorTree,
         want_disasm: bool,
     ) -> CodegenResult<CompiledCodeStencil>;