stage2: Runtime result of `@bitCast(u16, @Vector(16, bool))` is incorrect

### Zig Version

0.10.0-dev.4430+78778899

### Steps to Reproduce

```zig
const std = @import("std");
const expect = std.testing.expect;

test "bitcast vector to integer and back" {
    var x = @splat(16, true);
    x[1] = false;
    try expect(@bitCast(u16, x) == 0xfffd);
}
```

### Expected Behavior

Test should pass, like it does on stage1.

### Actual Behavior

Test fails