[llvm] [CodeGen] Remove static member function Register::isVirtualRegister. NFC (PR #127968)

[Christopher Di Bella via llvm-commits]

cjdb wrote:

This breaks a fair amount of downstream code. I've reverted this in 309e3ca08130f04ef6b0b320ee3e43575fa7bf99 and 08c69b2ef6eeba19956ad24fb7e9d29e9778cbaa; could we please have a period of time where `Register::isVirtualRegister` and `Register::isPhysicalRegister` are deprecated before being removed from the API?

https://github.com/llvm/llvm-project/pull/127968