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

[Nikita Popov via llvm-commits]

nikic wrote:

I wasn't able to confirm the compile-time impact because the LTO build crashed: https://llvm-compile-time-tracker.com/show_error.php?commit=428bd77c9a8807b3780d39ad5b5a757ac65dfaef (Presumably related to the lit test failures.)

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