Fix issue253.c:

Use def_op_num in process_insn_to_rename.
Permit mir as add file in runtests.sh.
Add issue253.c, add-issue253.mir, and issue253.expect.

c-tests/new/add-issue253.mir

@@ -0,0 +1,26 @@
+iteration:	module
+	export	iteration
+	import	print_identity
+	import	print_and_return_zero
+_proto_1:	proto	p, p:a0
+_proto_2:	proto	i32, p:a0
+iteration:	func	p, p:a0
+	local	i64:x2, i64:x1, i64:z1
+# 2 args, 4 locals
+	mov	x1, a0
+	mov	x2, 0
+L1:
+	bf	L3, x2
+	bt	L3, i64:(x2)
+L3:
+	call	_proto_1, print_identity, x2, x1
+	call	_proto_2, print_and_return_zero, z1, x2
+	bt	L4, z1
+	jmp	L5
+L4:
+	jmp	L1
+L5:
+	ret	x1
+ 	ret	x2
+endfunc
+endmodule

c-tests/new/issue253.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+void* print_identity (void* x) {
+  printf ("in print_identity: %p\n", x);
+  return x;
+}
+
+int print_and_return_zero (void* t) {
+  printf ("in print_and_return_zero: %p\n", t);
+  return 0;
+}
+
+extern void* iteration (void*);
+void* (*v) (void*) = iteration;
+
+int main (void) {
+  v ((void*) 0xdeadbeaf);
+  return 0;
+}