This patch adds a new special type of VPBasicBlock that wraps an
existing IR basic block. Recipes of the block get added before the
terminator of the wrapped IR basic block. Making it a subclass of
VPBasicBlock avoids duplicating various APIs to manage recipes in a
block, as well as makes sure the traversals filtering VPBasicBlocks
automatically apply as well.

Initially VPIRWrappedBlocks are only used for the pre-preheader
(wrapping the original preheader of the scalar loop).

As follow-up, this will be used to move more parts of the skeleton
inside VPlan, startingt with the branch and condition in the middle
block.

Note: This reqiores updating all VPlan-printing tests, which I will do
once we converge on a final version.

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