clang is suboptimal for `log(pow(x, y))` with fast-math

Test case:
https://godbolt.org/z/1fvYPxY37
```c
#include<math.h>

double test1(double x) { 
  return log(pow(x, -3)); 
}

double test2(double x) { 
  return -3 * log(x); 
}
```

My expectation is that the output of `test1` will be the same as `test2`.