make _V_mret_ unique by putting p.token_idx in it

vlib/compiler/tests/repeated_multiret_values_test.v

@@ -0,0 +1,15 @@
+// verify fix for #2913
+
+fn some_multiret_fn(a int, b int) (int, int) {
+	return a+1, b+1
+}
+
+fn test_repeated_multiple_multiret() {
+	a, b := some_multiret_fn(1,2)
+	assert a == 2
+	assert b == 3
+
+	c, d := some_multiret_fn(3,4)
+	assert c == 4
+	assert d == 5
+}