[clang] [llvm] [SelectionDAG][PowerPC] Add an intrinsic for memcmp. (PR #118178)

[Stefan Pintilie via cfe-commits]

stefanp-ibm wrote:

> You'll have to create an RFC on discourse if you want to add a new target-independent memory intrinsic.
> 
> Though if changing the name is the extent of the "special handling" you need, we probably shouldn't be adding an intrinsic for that.

We will want to add more handling than that in the future but it is not going to be soon. Very likely the additional handling will be for special cases where the `size_t num` parameter to `memcmp` is known at compile time. I wanted to add the intrinsic so that the backend knows that is isn't just looking at a regular function call where the function happened to be called `memcmp` but an actual call to THE `memcmp`. 

If this isn't the right way to do this what would you recommend? 


https://github.com/llvm/llvm-project/pull/118178