[VPlan] Properly handle sinking of replicate regions.

This patch updates the code that sinks recipes required for first-order
recurrences to properly handle replicate-regions. At the moment, the
code would just move the replicate recipe out of its replicate-region,
producing an invalid VPlan.

When sinking a recipe in a replicate-region, we have to sink the whole
region. To do that, we first need to split the block at the target
recipe and move the region in between.

This patch also adds a splitAt helper to VPBasicBlock to split a
VPBasicBlock at a given iterator.

Fixes PR50009.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D100751