checker, cgen: make the shared variable to be whole reassigned(fix #15649)

[shove70]

This PR:

1. Fixed V panics when using shared variable #15649
2. make the shared variable to be whole reassigned

fn test(shared values []int) {
	lock values {
		values = [2]
		println(values)
	}
}

fn main() {
	shared values := [3, 3, 3]
	test(shared values)
}

outputs:

[2]

---

Currently, V only supports the assignment of shared variables at definition time and the assignment of their members and elements, but not whole reassign to it self:

shared arr := [1, 2, 3]
lock arr {
    arr = [2]   // V panic and cgen error.
    arr[0] = 2  // this OK.
}

struct Foo{
    a int
}

shared st := Foo{}
lock st {
    st = Foo{}  // V panic and cgen error.
    st.a = 1    // this OK.
}

// Same goes for map.