checker, cgen: fix for c stmt with option or result calls (fix #19630)

[yuyi98]

This PR fix for c stmt with option or result calls (fix #19630).

• Fix for c stmt with option or result calls.
• Add test.

module main

fn foo() !int {
	return 5
}

fn main() {
	mut pos := 0

	for ; pos > 10; pos = foo()! {
		println(pos)
	}

	for pos = foo()!; pos > 10; {
		println(pos)
	}

	assert true
}

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

import rand
import math { abs }

const nmax = 20

fn ana(n int) f64 {
	return 1.0 // haven't started
}

fn avg(n int) f64 {
	tests := 1e4
	mut sum := 0
	for t := 0; t < tests; t++ {
		mut v := []bool{len: nmax, init: false}
		for x := 0; !v[x]; x = rand.intn(n) or { 0 } {
			v[x] = true
			sum++
		}
	}
	return f64(sum) / tests
}

fn main() {
	println(' N   average   analytical   (error)')
	println('=== ========= ============ =========')
	for n in 1 .. nmax + 1 {
		a := avg(n)
		b := ana(n)
		println('${n:3} ${a:9.4f} ${b:12.4f} (${(abs(a - b) / b * 100):6.2f}%)')
	}
	assert true
}

PS D:\Test\v\tt1> v run .
 N   average   analytical   (error)
=== ========= ============ =========
  1    1.0000       1.0000 (  0.00%)
  2    1.5047       1.0000 ( 50.47%)
  3    1.6645       1.0000 ( 66.45%)
  4    1.7550       1.0000 ( 75.50%)
  5    1.7972       1.0000 ( 79.72%)
  6    1.8323       1.0000 ( 83.23%)
  7    1.8565       1.0000 ( 85.65%)
  8    1.8808       1.0000 ( 88.08%)
  9    1.8890       1.0000 ( 88.90%)
 10    1.8931       1.0000 ( 89.31%)
 11    1.9026       1.0000 ( 90.26%)
 12    1.9171       1.0000 ( 91.71%)
 13    1.9179       1.0000 ( 91.79%)
 14    1.9242       1.0000 ( 92.42%)
 15    1.9329       1.0000 ( 93.29%)
 16    1.9389       1.0000 ( 93.89%)
 17    1.9399       1.0000 ( 93.99%)
 18    1.9449       1.0000 ( 94.49%)
 19    1.9498       1.0000 ( 94.98%)
 20    1.9534       1.0000 ( 95.34%)