v.gen.c: fix error of if expr (fix #7337)

[yuyi98]

This PR fix error of if expr (fix #7337).

• Fix error of if expr.
• Add test.

fn main() {
	mut a := true
	b := a && if true {
		a = false
		true
	} else {
		false
	}
	println(b)
	assert b == true
}

PS D:\Test\v\tt1> v -autofree run .
true

generated codes:

VV_LOCAL_SYMBOL void main__main(void) {
	bool a = true;
	bool _t1 = a;
	bool _t2; /* if prepend */
	if (true) {
		a = false;
		_t2 = true;
	} else {
		_t2 = false;
	}
		bool b =  _t1 &&  _t2;
	string _t3 = b ? _SLIT("true") : _SLIT("false"); println(_t3); string_free(&_t3);
	;
}

[spytheman]

That inverts the normal ordering. Try for example:

mut a := false

=> the inner if branches will still be executed, while normally, if x() && y() { } means that depending on the return value of x(), y() is short circuited and not evaluated.

[yuyi98]

That's true. For now, it's just an alternative to the ternary operator solution, there are some special cases.
Normally this would be a redundant execution, but the result should be correct.

[danieldaeschle]

I know nim solved this. It's possible to inspect their C generation.