[llvm] [VPlan] Constant-fold ActiveLaneMask intrinsics (PR #208852)

[Florian Hahn via llvm-commits]

================
@@ -1355,6 +1355,34 @@ VPIRValue *vputils::tryToFoldLiveIns(VPSingleDefRecipe &R,
     case Instruction::ExtractElement:
       assert(!Ops[0]->getType()->isVectorTy() && "Live-ins should be scalar");
       return Ops[0];
+    case VPInstruction::ActiveLaneMask:
----------------
fhahn wrote:

I think this case is currently not covered by tests.

Could you add something like the test below? I'd expect this to fold the first part, but not the second.

```
; opt -force-tail-folding-style=data -tail-folding-policy=must-fold-tail -force-vector-width=4
  -force-vector-interleave=2
define void @f(ptr noalias %dst, ptr noalias %src) {
entry:
  br label %loop

loop:
  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
  %p = getelementptr inbounds i64, ptr %src, i64 %iv
  %l = load i64, ptr %p, align 8
  %m = mul i64 %l, 3
  %q = getelementptr inbounds i64, ptr %dst, i64 %iv
  store i64 %m, ptr %q, align 8
  %iv.next = add nuw nsw i64 %iv, 1
  %ec = icmp eq i64 %iv.next, 7
  br i1 %ec, label %exit, label %loop

exit:
  ret void
}
```

https://github.com/llvm/llvm-project/pull/208852