LLVM cannot constant fold memchr with constant size

```c++
void* test1(char* first, char val) {
  return __builtin_memchr(first, val, 2);
}
```
always generates a call to `memchr`, even though it could just check the two chars. I can't really see a case where calling `memchr` is faster than just checking the two chars. For larger N it would also be interesting to use vector comparisons for this instead of calling a function.
