Reapply "[VPlan] Set branch weight metadata on middle term in VPlan (NFC) (#143035)"

This reverts commit 0604dc199c019b23746f4a54885ba0c75569cdae.

The recommitted version addresses post-commit comments and adjusts the
place the branch weights are added. It now runs before VPlans are optimized
for VF and UF, which may remove the vector loop region, causing a crash
trying to get the middle block after that. Test case added in
72f99b75afc12bb.

Original message:
Manage branch weights for the BranchOnCond in the middle block in VPlan.
This requires updating VPInstruction to inherit from VPIRMetadata, which
in general makes sense as there are a number of opcodes that could take
metadata.

There are other branches (part of the skeleton) that also need branch
weights adding.

PR: https://github.com/llvm/llvm-project/pull/143035

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