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

[via llvm-commits]

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@@ -187,6 +187,54 @@ define void @test_write_before_memset_in_both_regions(ptr %result) {
   ret void
 }
 
+define void @test_offset_memset(ptr %result) {
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dianqk wrote:

```suggestion
define void @test_negative_offset_memset(ptr %result) {
```

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