[SimplifyLibCalls] Transform memchr(STR, C, N) to chain of ORs

Motivation:

```
#include <string_view>

size_t findFirst_ABCDEF(std::string_view sv) {
  return sv.find_first_of("ABCDEF");
}
```

memchr("ABCDEF", C,  6) != NULL -> (C == 'A' || C == 'B' || C == 'C' || C == 'D' || C == 'E' || C == 'F') != 0

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D128011