[VPlan] Create AVL as a phi from TC -> 0 with EVL tail folding (#151481)

This implements the first half of #151459, by changing the AVL so it's
no longer computed as `trip-count - EVL-based IV`, but instead a
separate scalar phi that is decremented by EVL each iteration.

This shortens the dependency chain for computing the AVL and should
eventually allow us to convert the branch condition to `branch-count
avl-next, 0`.

`simplifyBranchConditionForVFAndUF` had to be updated to prevent a
regression because this introduces a VPPhi in the header block.

0 files changed, 0 insertions, 0 deletions