[llvm] [LoopIdiom] Select llvm.experimental.memset.pattern intrinsic rather than memset_pattern16 libcall (PR #126736)

[Philip Reames via llvm-commits]

preames wrote:

> The bigger one is I see some examples with repeated memset_pattern16 calls that wasn't there before. After looking at some of the others, I'm now isolating a smaller example.

I wrote a bunch of test cases to see if I could identity your problem.  I believe this is a consequence of the second parameter (the value param) not being known to be readnone.  There might be other cases, but I can definitely see this case failing to fold in DSE and GVN.  

https://github.com/llvm/llvm-project/pull/126736