[llvm] [LiveDebugValues] Use getNumSupportedRegs to reduce compile time, NFCI (PR #114944)

[Jeremy Morse via llvm-commits]

jmorse wrote:

(While this is marked draft, reducing the known set of register that might need tracking is a solid plan, so this is a good direction to take).

https://github.com/llvm/llvm-project/pull/114944