LibC: Relax memmove() to memcpy() in more cases

`memcpy()` and `memmove()` are functionally equivalent if the source and
destination memory regions do not overlap. If this is the case, we
should prefer `memcpy()` as it's implemented in a simpler and faster
way. As our `memcpy()` starts copying from the lowest address first,
relaxing a `memmove()` call to `memcpy()` is safe if the destination:
- starts at a lower address than the source
- starts at a higher address than the source's end

Userland/Libraries/LibC/string.cpp

@@ -167,6 +167,8 @@ void* memset(void* dest_ptr, int c, size_t n)
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/memmove.html
 void* memmove(void* dest, void const* src, size_t n)
 {
+    if (dest < src)
+        return memcpy(dest, src, n);
     if (((FlatPtr)dest - (FlatPtr)src) >= n)
         return memcpy(dest, src, n);