checker: fix immutable to mutable reference

[felipensp]

Fix #22653

Fixes cases like this:

fn main() {
	a := ['foo', 'bar']
	mut b := []string{}
	b = if true { a } else { a } 	
	b[0] = 'bar'
	dump(a) // ['bar', 'bar']
}

_{Huly®: V_0.6-21112}