[SimplifyCFG] Optimize CFG when null is passed to a function with nonnull argument

Example:

```
__attribute__((nonnull,noinline)) char * pinc(char *p)  {
  return ++p;
}

char * foo(bool b, char *a) {
       return pinc(b ? 0 : a);

}
```

optimize to

```
char * foo(bool b, char *a) {
       return pinc(a);

}
```

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D94180