[llvm] [VPlan] Convert to concrete recipes before dissolving loop regions. NFCI (PR #141999)

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ayalz wrote:

Post commit comment: the latch could be retrieved w/ or w/o regions similar to retrieving the header via `getFirstLoopHeader()`, using VPDT. Otherwise if the pass depends on regions would be good to have a comment stating so.

https://github.com/llvm/llvm-project/pull/141999