[VPlan] Fix LastActiveLane assertion on scalar VF (#167897)

For a scalar only VPlan with tail folding, if it has a phi live out then
legalizeAndOptimizeInductions will scalarize the widened canonical IV
feeding into the header mask:

    <x1> vector loop: {
      vector.body:
        EMIT vp<%4> = CANONICAL-INDUCTION ir<0>, vp<%index.next>
        vp<%5> = SCALAR-STEPS vp<%4>, ir<1>, vp<%0>
        EMIT vp<%6> = icmp ule vp<%5>, vp<%3>
        EMIT vp<%index.next> = add nuw vp<%4>, vp<%1>
        EMIT branch-on-count vp<%index.next>, vp<%2>
      No successors
    }
    Successor(s): middle.block

    middle.block:
      EMIT vp<%8> = last-active-lane vp<%6>
      EMIT vp<%9> = extract-lane vp<%8>, vp<%5>
    Successor(s): ir-bb<exit>

The verifier complains about this but this should still generate the
correct last active lane, so this fixes the assert by handling this case
in isHeaderMask. There is a similar pattern already there for
ActiveLaneMask, which also expects a VPScalarIVSteps recipe.

Fixes #167813

0 files changed, 0 insertions, 0 deletions