cgen: fix `$if T !is` (#8311)

vlib/v/gen/comptime.v

@@ -268,7 +268,8 @@ fn (mut g Gen) comp_if_expr(cond ast.Expr) {
 						// this is only allowed for generics currently, otherwise blocked by checker
 						exp_type = g.unwrap_generic(left.typ)
 					}
-					g.write('$exp_type == $got_type')
+					op := if cond.op == .key_is { '==' } else { '!=' }
+					g.write('$exp_type $op $got_type')
 				}
 				.eq, .ne {
 					// TODO Implement `$if method.args.len == 1`

vlib/v/tests/comptime_if_is_test.v

@@ -0,0 +1,15 @@
+fn f<T>() int {
+	$if T is int {
+		return 1
+	}
+	$if T !is int {
+		return -1
+	}
+	assert false
+	return 0
+}
+
+fn test_generic_is() {
+	assert f<int>() == 1
+	assert f<bool>() == -1
+}