[llvm] Remove SmallSet from MachineInstr.h

`MachineInstr.h` is a commonly included file and this includes
`llvm/ADT/SmallSet.h` for one function `getUsedDebugRegs()`, which is
used only in one place.

According to `ClangBuildAnalyzer` (run solely on building LLVM, no other
projects) the second most expensive template to instantiate is the
`SmallSet::insert` method used in the `inline` implementation in
`getUsedDebugRegs()`:

```
**** Templates that took longest to instantiate:
554239 ms: std::unordered_map<int, int> (2826 times, avg 196 ms)
521187 ms: llvm::SmallSet<llvm::Register, 4>::insert (930 times, avg 560
       ms)
...
```

By removing this method and putting its implementation in the one call
site we greatly reduce the template instantiation time and reduce the
number of includes.

When copying the implementation, I removed a check on `MO.getReg()` as
this is checked within `MO.isVirtual()`.

Differential Revision: https://reviews.llvm.org/D157720