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

[Alan Phipps via llvm-commits]

evodius96 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.

Yes, effectively -- at least our downstream embedded toolchain for baremetal targets uses a stripped down compiler-rt.

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