checker: fix if cond with alias (fix #17818)

[yuyi98]

This PR fix if cond with alias (fix #17818).

• Fix if cond with alias.
• Add test.

type BOOL = bool

fn example() BOOL {
	return true
}

fn main() {
	if example() {
		println('Should work, or not?')
		assert true
	} else {
		assert false
	}
}

PS D:\Test\v\tt1> v run .
Should work, or not?