[clang] [libc] [llvm] [clang-tools-extra] [llvm-exegesis] Add middle half repetition mode (PR #77020)

[Aiden Grossman via cfe-commits]

boomanaiden154 wrote:

@legrosbuffle This should be ready to again when you have a chance to take a look. Thanks!

https://github.com/llvm/llvm-project/pull/77020