[llvm] [SelectionDAG] Fix unsafe cases for loop.dependence.{war/raw}.mask (PR #168565)

[Benjamin Maxwell via llvm-commits]

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@@ -1569,8 +1569,21 @@ enum NodeType {
   GET_ACTIVE_LANE_MASK,
 
   // The `llvm.loop.dependence.{war, raw}.mask` intrinsics
-  // Operands: Load pointer, Store pointer, Element size
+  // Operands: Load pointer, Store pointer, Element size, Lane offset
   // Output: Mask
+  //
+  // Note: The semantics of these opcodes differ slightly from the intrinsics.
+  // Wherever "lane" (meaning lane index) occurs in the intrinsic definition, it
+  // is replaced with (lane + lane_offset) for the ISD opcode.
+  //
+  //  E.g., for LOOP_DEPENDENCE_WAR_MASK:
+  //    `(ptrB - ptrA) >= elementSize * lane`
+  //  Becomes:
+  //    `(ptrB - ptrA) >= elementSize * (lane + lane_offset)`
----------------
MacDue wrote:

I took this from the LangRef, but I think it might actually need to be:
```suggestion
  //  E.g., for LOOP_DEPENDENCE_WAR_MASK:
  //    `(ptrB - ptrA) > elementSize * lane`
  //  Becomes:
  //    `(ptrB - ptrA) > elementSize * (lane + lane_offset)`
```
Which I think matches whilewr (and the current lowering), I think the same may be true to the `.raw` mask.

https://github.com/llvm/llvm-project/pull/168565