[llvm] [KeyInstr] Add bitcode support (PR #144102)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

> Looks good; do you have any size comparisons for bitcode files as a result of this? All the DILocations are bigger (regardless of KI mode), but on the other hand the debug-loc-again optimisation will avoid paying for that for every instruction.

Build with support enabled and the feature on by default: [compile time tracker link](https://llvm-compile-time-tracker.com/compare.php?from=f286f02be30632605d99b850a8159019852cd7c0&to=fdb5f8048ac8ae713392260fee52a5e7ecdcc812&stat=size-file&details=on).

> but on the other hand the debug-loc-again optimisation will avoid paying for that for every instruction.

Yes and no. It's less likely that you find identical debug-locs, considering both group and rank. I considered whether it might be reasonable to change the debug-loc-again to ignore KeyInstructions info in the comparison and encode it. I didn't end up pursuing the idea, but I can look into it if you feel it's needed.

https://github.com/llvm/llvm-project/pull/144102