parser: fix invalid warning 'unused' for mut var in if/match cond(fix#19539)

[shove70]

Fixed #19539

struct Foo {
	value int = 0xf00
}

struct Bar {}

type FooBar = Bar | Foo

fn main() {
	mut foobar := FooBar(Bar{})
	match mut foobar {
		Foo {
			println('foo')
		}
		else {
			println('bar')
		}
	}
}

struct Foo {
	value int = 0xf00
}

struct Bar {}

type FooBar = Bar | Foo

fn main() {
	mut foobar := FooBar(Bar{})
	if mut foobar is Foo {
		println('foo')
	} else {
		println('bar')
	}
}

Invalid warnings have been removed.