[VPlan] Compute branch costs in VPlan. (#234377)
Compute the costs of branches in VPlan, replacing the legacy logic in
precomputeCost. We now compute the cost of branches in replicate regions
directly in VPRegionBlock::cost and VPBranchOnMaskRecipe::cost,
including the cost of the unconditional branches from the conditionally
execute block, as well between plain VPBBs (this is currently not used,
but in preparation for computing scalar costs + supporting conditional
blocks in plans).
The cost changes in tests are because we previously
* missed replicate regions (e.g. tail-folding in a loop without any
branches),
* overcounted conditional branches (e.g. because replicate regions could
be merged in VPlan, chains of conditional branches resulting in a single
replicate region)
PR: https://github.com/llvm/llvm-project/pull/234377
