cgen: `or {}` block containing a comptime `$if/$else` emits an undeclared result temp (`_t3`)

[medvednikov]

V version: V 0.5.2 (master)
OS: Linux (reported via bugs.vlang.io)

Description

An or {} block whose body is a compile-time $if / $else emits an undeclared result temp: the branch bodies assign to and reference _t3, but _t3 is never declared, so the generated C fails with use of undeclared identifier '_t3'.

Reproduction

module main

fn g(i int) !int {
	return error('!')
}

x := g(1) or {
	$if !windows {
		panic(err)
	} $else {
		u32(0)
	}
}

Actual (generated C fails to compile)

error: use of undeclared identifier '_t3'
    _t3 = builtin___v_panic(builtin__IError_str(err));
    ...
    _t3;

The or {} result temp _t3 is used in the $if/$else branch bodies but never declared.

Expected

Compiles; the or {} block's result temp is declared for the comptime-$if body.

Reported via bugs.vlang.io (also seen inside a larger real program)

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