[llvm] [MC/DC][Coverage] Split out Read-modfy-Write to rmw_or(ptr, i8) (PR #96040)

[Ellis Hoag via llvm-commits]

ellishg wrote:

Is MC/DC expected to ever be used without a profile runtime library? If not, you could create a new function in compiler-rt instead of creating it in IR.

https://github.com/llvm/llvm-project/pull/96040