preopt: use replaced arg after having replaced BinaryImm

when replacing BinaryImm, we use the prior arg, but later use the arg
that was replaced when writing an alias if we can determine the new op
is actually equivalent to a simple copy

cranelift/codegen/src/simple_preopt.rs

@@ -585,6 +585,7 @@ fn simplify(pos: &mut FuncCursor, inst: Inst) {
         InstructionData::BinaryImm { opcode, arg, imm } => {
             let ty = pos.func.dfg.ctrl_typevar(inst);
 
+            let mut arg = arg;
             let mut imm = imm;
             match opcode {
                 Opcode::IaddImm
@@ -613,12 +614,13 @@ fn simplify(pos: &mut FuncCursor, inst: Inst) {
                                         _ => panic!("can't happen"),
                                     };
                                     let new_imm = immediates::Imm64::from(new_imm);
-                                    let arg = *prev_arg;
+                                    let new_arg = *prev_arg;
                                     pos.func
                                         .dfg
                                         .replace(inst)
-                                        .BinaryImm(opcode, ty, new_imm, arg);
+                                        .BinaryImm(opcode, ty, new_imm, new_arg);
                                     imm = new_imm;
+                                    arg = new_arg;
                                 }
                             }
                         }

cranelift/filetests/filetests/simple_preopt/simplify.clif

@@ -278,3 +278,16 @@ ebb0:
 ; nextln:     v2 = sextend.i64 v3
 ; nextln:     return v2
 ; nextln: }
+
+function %add_imm_fold(i32) -> i32 {
+ebb0(v0: i32):
+  v1 = iadd_imm v0, 42
+  v2 = iadd_imm v1, -42
+  return v2
+}
+; sameln: function %add_imm_fold(i32)
+; nextln: ebb0(v0: i32):
+; nextln:    v2 -> v0
+; nextln:    v1 = iadd_imm v0, 42
+; nextln:    nop
+; nextln:    return v2