[llvm] [llvm] Improve implementation of StringRef::find_last_of and cie (PR #71865)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Does this micro-optimization adversely affect cases that aren't covered by it (eg: does checking for length 2 slow down non-length-2 cases perceptibly?)

https://github.com/llvm/llvm-project/pull/71865