checker: fix type mismatch checking for assignments with generics(fix#20298)

[shove70]

1. Fixed ./v test vlib/v/tests/generics_chans_select_test.v fails #20298
2. Add tests.

This issue(20298) contains several issues. This PR starts with a simple solution to a type checking problem.
Then split a new issue, and resolve it.

The new issue is a CallExpr may have 2 or block{} :

type Fn = fn ()

struct Struct {
    method ?Fn
}

fn main() {
    s := Struct {
        method: fn () {}
    }

    s.method or_1 {} (args...) or_2 {}
}

or_1 {} for ?Fn
or_2 {} for method call.

[shove70]

new issue:
#20328

[spytheman]

Excellent work.