make _V_mret_ unique by putting p.token_idx in it

vlib/compiler/tests/reusable_mut_multiret_values_test.v

@@ -0,0 +1,27 @@
+// verify fix for #2913
+
+fn some_multiret_fn(a int, b int) (int, int) {
+	return a+1, b+1
+}
+
+fn test_reuse_multiple_multiret() {
+	mut c, mut d := some_multiret_fn(4,10)
+
+	mut a, mut b := some_multiret_fn(c,d)
+	assert a == c+1
+	assert b == d+1
+
+	for i in 1..10 {
+		c += i
+		d += i
+		a, b = some_multiret_fn(c,d)
+		assert a == c+1
+		assert b == d+1
+
+		c += i+1
+		d += i+1
+		a, b = some_multiret_fn(c,d)
+		assert a == c+1
+		assert b == d+1
+	}
+}