[llvm] [LiveDebugValues] Generalize approach to find stack units, NFC (PR #124862)

[Emma Pilkington via llvm-commits]

epilk wrote:

@jmorse: We hit a significant compile time issue with this function using InstrRefBased LDV on AMD's downstream fork on AMDGPU (which has a lot more subregister idxes), we're planning on upstreaming at some point but not immediately. Would you rather fix this now or wait until there is an upstream justification beyond just being more general?

https://github.com/llvm/llvm-project/pull/124862