gen: fix generating `$if expr || expr` (#9270)

vlib/v/gen/c/comptime.v

@@ -286,7 +286,7 @@ fn (mut g Gen) comp_if_cond(cond ast.Expr) bool {
 					l := g.comp_if_cond(cond.left)
 					g.write(' $cond.op ')
 					r := g.comp_if_cond(cond.right)
-					return l && r
+					return if cond.op == .and { l && r } else { l || r }
 				}
 				.key_is, .not_is {
 					left := cond.left

vlib/v/tests/comptime_if_is_test.v

@@ -13,3 +13,17 @@ fn test_generic_is() {
 	assert f<int>() == 1
 	assert f<bool>() == -1
 }
+
+fn g<T>(t T) int {
+	$if T is byte || T is i8 {
+		return 1
+	}
+	return 2
+}
+
+fn test_is_or() {
+	assert g(byte(1)) == 1
+	assert g(i8(1)) == 1
+	assert g(1) == 2
+}
+