[llvm] [VPlan] Allow tail folding with uncountable early exits (PR #174974)

[Shih-Po Hung via llvm-commits]

================
@@ -3952,6 +3946,18 @@ void VPlanTransforms::handleUncountableEarlyExit(VPBasicBlock *EarlyExitingVPBB,
                               ? CondOfEarlyExitingVPBB
                               : Builder.createNot(CondOfEarlyExitingVPBB);
 
+  // Create a mask and predicate any "exited" lanes in successor blocks.
+  VPValue *FirstActiveLane =
+      Builder.createNaryOp(VPInstruction::FirstActiveLane, {CondToEarlyExit},
+                           DebugLoc::getUnknown(), "first.active.lane");
----------------
arcbbb wrote:

   To avoid poison when the trip count isn't a multiple of VF (tail-foled loops), I am thinking about an EarlyExit recipe like this:
```
   %EarlyExitMask = EarlyExit %CondToEarlyExit
   %FirstActiveLane = first.active.lane %EarlyExitMask
   ...
   %EarlyExit = AnyOf %EarlyExitMask
```
   Later, `(EarlyExit %cond)` need to be lowered into:
   `(logical-and %cond, (active-lane-mask 0, (extractvalue %ffload, 1)))`
if the user is a masked load/store or branch.

https://github.com/llvm/llvm-project/pull/174974