cgen: fix cast in the index of reference fixed array (fix #13128)

[yuyi98]

This PR fix cast in the index of reference fixed array (fix #13128).

• Fix cast in the index of reference fixed array.
• Add test.

type Entity = int

struct Component {
	vals [256]int
}

fn (c Component) get_broken(e Entity) &int {
	return &c.vals[int(e)]
}

fn main() {
	c := Component{}
	a := c.get_broken(Entity(10))
	println(*a)
	assert *a == 0
}

PS D:\Test\v\tt1> v run .
0