checker, cgen: fix auto dereference mut variable in if expr (fix #21309)

[yuyi98]

This PR fix auto dereference mut variable in if expr (fix #21309).

• Fix auto dereference mut variable in if expr.
• Add test.

struct Foo {
	str string
}

fn (mut f Foo) foo(f2 Foo) string {
	return (if f.str != '' {
		f
	} else {
		f2
	}).str
}

fn main() {
	mut foo := Foo{}
	dump(foo.foo(Foo{ str: 'a' }))
	assert foo.foo(Foo{ str: 'a' }) == 'a'
}

PS D:\Test\v\tt1> v run .
[.\\tt1.v:15] foo.foo(main.Foo{....}): a