[llvm] [MemCpyOpt] Calculate the offset value to forward `memcpy` (PR #87190)

[Yingwei Zheng via llvm-commits]

================
@@ -1152,11 +1188,12 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
   //
   // TODO: If the code between M and MDep is transparent to the destination "c",
   // then we could still perform the xform by moving M up to the first memcpy.
-  // TODO: It would be sufficient to check the MDep source up to the memcpy
-  // size of M, rather than MDep.
-  if (writtenBetween(MSSA, BAA, MemoryLocation::getForSource(MDep),
-                     MSSA->getMemoryAccess(MDep), MSSA->getMemoryAccess(M)))
+  if (writtenBetween(MSSA, BAA, MCopyLoc, MSSA->getMemoryAccess(MDep),
+                     MSSA->getMemoryAccess(M))) {
+    if (MForwardOffset > 0 && CopySource->use_empty())
+      cast<Instruction>(CopySource)->eraseFromParent();
----------------
dtcxzyw wrote:

Should we remove the temporary GEP here?
```
bool UseMemMove = false;
  if (isModSet(BAA.getModRefInfo(M, MemoryLocation::getForSource(MDep)))) {
    // Don't convert llvm.memcpy.inline into memmove because memmove can be
    // lowered as a call, and that is not allowed for llvm.memcpy.inline (and
    // there is no inline version of llvm.memmove)
    if (isa<MemCpyInlineInst>(M))
      return false; --> Remove CopySource if unused
    UseMemMove = true;
  }
```

https://github.com/llvm/llvm-project/pull/87190