cgen: fix codegen for method call on rangeexpr

[felipensp]

Fix #12610

type Buffer = []byte

pub fn (mut sb Buffer) vbytes() string {
	return sb[0..sb.len].a().str()
}

pub fn (mut sb Buffer) a() Buffer {
	return sb
}

fn test_main() {
	mut b := Buffer([]byte{cap: 10})
	b << 1
	b << 2
	assert b.vbytes() == 'Buffer([1, 2])'
}

[spytheman]

Excellent work.