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

[Nikita Popov via llvm-commits]

nikic wrote:

Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=a256e89fd1b9f42d3f29df42d4f21ea823476ff3&to=a3ffd3d22936650dc23828b92afdb9e1b4d24f1e&stat=instructions:u

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