[VPlan] Extend induction cost test coverage (NFC). (#221323)
Widen induction-cost.ll's filter to cover all induction recipes and the
canonical IV increment, run it for two VFs, and add the FP and pointer
induction shapes it was missing entirely: scalarized
FAdd/FSub/invariant-step inductions, a dead induction in the vector
loop, one with both wide and scalar users, one inside a replicate
region, a fully unrolled one, and FP-plus-pointer and pointer-only
loops.