[clang] [clang][bytecode] Split `InterpFrame::shrink()`/`peekData()` into slow and fast paths (PR #225761)

[Timm Baeder via cfe-commits]

tbaederr wrote:

This saves 4.5% in `APFloat.cpp` and 6.5% in `AttrDocTable.cpp`: https://llvm-compile-time-tracker.com/compare_clang.php?from=64457d57b773f9d1179ca51a17ce0a9fc6fe3b86&to=36dcac396fb874bbe3978ef4c7079ea105b1eef5&stat=instructions%3Au&sortBy=interestingness

https://github.com/llvm/llvm-project/pull/225761