[llvm] [memcpyopt] handle memcpy from memset in more cases (PR #140954)

[via llvm-commits]

================
@@ -18,13 +18,13 @@ define void @test(ptr %src, i8 %c, i64 %size) {
   ret void
 }
 
-; Differing sizes, so left as it is.
+; Differing sizes, but would be UB if size1 > size2
----------------
dianqk wrote:

Why is this (immediate) UB?

https://github.com/llvm/llvm-project/pull/140954