[llvm] [VPlan] Add new VPInstruction ocpode for header mask. (PR #89603)

[via llvm-commits]

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@@ -1180,6 +1180,9 @@ class VPInstruction : public VPRecipeWithIRFlags {
     // operand). Only generates scalar values (either for the first lane only or
     // for all lanes, depending on its uses).
     PtrAdd,
+    // An abstract representation of the vector loops header mask, to be lowered
+    // later depending on target preference.
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ayalz wrote:

```suggestion
    // later depending on target preference. Relevant only when the header may have a partial mask, i.e., when tail folding. A mask known to always be full is represented by null, w/o a HeaderMask recipe. A header mask may not be empty.
```

https://github.com/llvm/llvm-project/pull/89603