[llvm] [VPlan] Don't bail creating interleave group if it has dead members (PR #208415)

[Mel Chen via llvm-commits]

================
@@ -3844,6 +3846,10 @@ void VPlanTransforms::createInterleaveGroups(
 
     Instruction *IRInsertPos = IG->getInsertPos();
     auto *InsertPos = IRMemberToRecipe.lookup(IRInsertPos);
+    if (!InsertPos) {
+      InsertPos = Start;
+      IRInsertPos = StartMember;
+    }
----------------
Mel-Chen wrote:

There's an issue here. The order of InsertPos and group members is actually independent; InsertPos depends on the program order. Therefore, when the InsertPos recipe is dead, we need to find a new InsertPos recipe instead of just setting it to the first alive member. Maybe we can use properlyDominates to find the new InsertPos. Like:
``` c++
if (!InsertPos) {
    InsertPos = Start;
    for (unsigned I = IG->getIndex(StartMember) + 1; I < IG->getFactor(); ++I) {
        Instruction *Member = IG->getMember(I);
        if (!Member)
            continue;
        auto *MemberR = IRMemberToRecipe.lookup(Member);
        if (!MemberR)
            continue;
        if (VPDT.properlyDominates(MemberR, InsertPos))
            InsertPos = MemberR;
    }
    IRInsertPos = cast<VPWidenMemoryRecipe>(InsertPos)->getIngredient();
}
```
and add a test case where the load order is: member 2 (Dead InsertPos) -> member 1 -> member 0.

https://github.com/llvm/llvm-project/pull/208415