[llvm] [VPlan] Add extra checks for LoopForBB. NFC. (PR #132306)

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offsake wrote:

Hello @fhahn ,

LoopBlocksRPO RPO(TheLoop); code might guarantee that all visited blocks have an associated loop. It means that the check below is redundant, which I removed in the second commit.

https://github.com/llvm/llvm-project/pull/132306