Fix operator[] for vector2d and vector3d being potentially UB (#15977)

We don't have a C++ expert on hand, but taking a pointer to one member
and expecting to access another by an offset is very fishy:
- for one, there could theoretically be padding
- the compiler might assume that we are only writing to that first member

The new code has shown to be free for constant parameter values.
Non-constant ones cause the assembly to have branches (why?), but we don't
use that much.

irr/src/os.h

@@ -10,22 +10,6 @@
 #include "ILogger.h"
 #include "ITimer.h"
 
-// CODE_UNREACHABLE(): Invokes undefined behavior for unreachable code optimization
-#if defined(__cpp_lib_unreachable)
-#include <utility>
-#define CODE_UNREACHABLE() std::unreachable()
-#elif defined(__has_builtin)
-#if __has_builtin(__builtin_unreachable)
-#define CODE_UNREACHABLE() __builtin_unreachable()
-#endif
-#elif defined(_MSC_VER)
-#define CODE_UNREACHABLE() __assume(false)
-#endif
-
-#ifndef CODE_UNREACHABLE
-#define CODE_UNREACHABLE() (void)0
-#endif
-
 namespace irr
 {