[clang] [clang][bytecode] Optimize InterpStack (PR #159400)

[Timm Baeder via cfe-commits]

tbaederr wrote:

https://llvm-compile-time-tracker.com/compare.php?from=a0162219165b94b950fe10547307378c0a306b74&to=03ef1e9181aaec7f0601736a42e5db842b0d4e4f&stat=instructions:u

https://github.com/llvm/llvm-project/pull/159400