Simplify branch arguments after splitting EBB arguments.

The EBB argument splitting may generate concat-split dependencies when
it repairs branch arguments in EBBs that have not yet been fully
legalized. Add a branch argument simplification step that can resolve
these dependency chains.

This means that all split and concatenation instructions will be dead
after legalization for types that have no legal instructions using them.

lib/cretonne/src/legalizer/mod.rs

@@ -54,6 +54,10 @@ pub fn legalize_function(func: &mut Function, cfg: &mut ControlFlowGraph, isa: &
                 continue;
             }
 
+            if opcode.is_branch() {
+                split::simplify_branch_arguments(&mut func.dfg, inst);
+            }
+
             match isa.encode(&func.dfg, &func.dfg[inst]) {
                 Ok(encoding) => *func.encodings.ensure(inst) = encoding,
                 Err(action) => {