checker: fix return map index with or_block (fix #20391)

[yuyi98]

This PR fix return map index with or_block (fix #20391).

• Fix return map index with or_block.
• Add test.

fn get_value1(m map[string]int, key string) ?int {
	return m[key] or { none }
}

fn get_value2(m map[string]int, key string) !int {
	return m[key] or { error('not found') }
}

fn main() {
	m := {
		'aaa': 111
	}
	if ret1 := get_value1(m, 'aaa') {
		println(ret1)
		assert ret1 == 111
	} else {
		assert false
	}

	if ret2 := get_value2(m, 'aaa') {
		println(ret2)
		assert ret2 == 111
	} else {
		assert false
	}
}

PS D:\Test\v\tt1> v run .
111
111

[spytheman]

I can not decide on whether the example should work.

From one point of view:

return expr

should behave exactly the same as:

tmp := expr
return tmp

and you can not do tmp := expr_res or { error('x') },
but you can do: tmp := expr_opt or { none }.

From another point of view, it seems clear what is wanted by just:

fn f() ?Type { return expr or { none } }

and by:

fn f() !Type { return expr or { error('x') } }

@medvednikov, what do you think, should V work like in this PR?

[spytheman]

(It is excellent work, anyways, whatever is decided.)

[yuyi98]

Here is to omit return, equivalent to return none and return error('....').

[medvednikov]

I think this should be merged.