checker: fix generic comparison for conditional assignment (fix #19398)

[yuyi98]

This PR fix generic comparison for conditional assignment (fix #19398).

• Fix generic comparison for conditional assignment.
• Add test.

fn clip[N](x []N, min N, max N) []N {
	mut result := []N{cap: x.len}
	for value in x {
		result << if value < min {
			min
		} else if value > max {
			max
		} else {
			value
		}
	}
	return result
}

fn main() {
	a := [1, 2, 3, 4, 5]
	dump(a)
	b := clip(a, 2, 4)
	dump(b)
	assert b == [2, 2, 3, 4, 4]
}

PS D:\Test\v\tt1> v run .    
[.\\tt1.v:17] a: [1, 2, 3, 4, 5]
[.\\tt1.v:19] b: [2, 2, 3, 4, 4]

[spytheman]

Excellent. Thank you!