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/context.rs

@@ -140,7 +140,7 @@ impl Context {
 
         self.optimize(isa)?;
 
-        isa.compile_function(&self.func, self.want_disasm)
+        isa.compile_function(&self.func, &self.domtree, self.want_disasm)
     }
 
     /// Optimize the function, performing all compilation steps up to