[VPlan] Move dropping of poison flags to VPlanTransforms. (NFC)

Move collectPoisonGeneratingFlags from InnerLoopVectorizer to
VPlanTransforms and also update its name. collectPoisonGeneratingFlags
already directly drops poison-generating flags, not only collecting it.
This means it is more appropriate to integerate it directly into the
VPlan transform pipeline.

The current implementation still calls back to legal to check if a block
needs predication, which should be improved in the future.