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

[Matthias Braun via llvm-commits]

MatzeB wrote:

At a first glance it seems like users in `Register` should always call `Register::index2StackSlot` / `Register::stackSlot2Index` and if someone were to pass `NO_STACK_SLOT` to it, it would overflow anyway.

`INT_MAX` seems fine to me as there's no test failures...

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