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

[via llvm-commits]

serge-sans-paille wrote:

@aganea / @joker-eph / @kazutakahirata : I actually switched the implementation to something that's almost as efficient (no vector load required) and works across all architecture. (Not to mention it also increases my geekness karma)

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