[llvm] [VirtRegMap] Replace a single value enum with a static constexpr member variable. NFC (PR #109010)

[Quentin Colombet via llvm-commits]

qcolombet wrote:

> @MatzeB do you think it's possible to use INT_MAX instead of our own number here?

I don't remember if this value needs to match what we have for describe registers, but if so we need to stick to it.
See `llvm/include/llvm/CodeGen/Register.h`
```
  //   [2^30;2^31) Stack slots. (Rarely used.)
```

@topperc double check if it needs to be connected and if not `INT_MAX` may be fine.

https://github.com/llvm/llvm-project/pull/109010