[InstrProf] Fix warning about converting double to float

In https://reviews.llvm.org/D147812 I introduced the class
`BalancedPartitioning` and it seemed to trigger a warning in flang

```
C:\Users\buildbot-worker\minipc-ryzen-win\flang-x86_64-windows\llvm-project\llvm\include\llvm/Support/BalancedPartitioning.h(89): warning C4305: 'initializing': truncation from 'double' to 'float'
```

For good measure, I converted all double literals to floats. This should
be a NFC.