Fix isplit legalization

cranelift/codegen/src/legalizer/mod.rs

@@ -65,21 +65,45 @@ fn legalize_inst(
             _ => panic!("Expected isplit: {}", pos.func.dfg.display_inst(inst, None)),
         };
 
+        let should_replace = match pos.func.dfg.value_def(arg) {
+            ir::ValueDef::Result(inst, num) => {
+                if let ir::InstructionData::Binary { opcode, args, .. } = pos.func.dfg[inst] {
+                    opcode == ir::Opcode::Iconcat
+                } else {
+                    // `arg` was not created by an `iconcat` instruction. Don't try to resolve it,
+                    // as otherwise `split::isplit` will re-insert the original `isplit`, causing
+                    // an endless loop.
+                    false
+                }
+            }
+            ir::ValueDef::Param(ebb, num) => true,
+        };
+
+        if should_replace {
             let res = pos.func.dfg.inst_results(inst).to_vec();
             assert_eq!(res.len(), 2);
             let (resl, resh) = (res[0], res[1]); // Prevent borrowck error
 
+            dbg!(pos.position());
+
+            // Remove old isplit
+            pos.func.dfg.clear_results(inst);
+            pos.remove_inst();
+
+            dbg!(pos.position());
+
             let curpos = pos.position();
             let srcloc = pos.srcloc();
             let (xl, xh) = split::isplit(pos.func, cfg, curpos, srcloc, arg);
 
-        pos.func.dfg.clear_results(inst);
-        pos.remove_inst();
             pos.func.dfg.change_to_alias(resl, xl);
             pos.func.dfg.change_to_alias(resh, xh);
 
+            dbg!(&pos.func);
+
             return true;
         }
+    }
 
     match pos.func.update_encoding(inst, isa) {
         Ok(()) => false,