[llvm] [VPlan] First step towards VPlan cost modeling. (PR #92555)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Addressed comments and also added 2 more cases where costs need to be pre-computed to match legacy cost modeling:

1) any-of reductions: with the latest VPlan codegen, the original select will be replaced, requiring pre-computing the cost
2) the legacy cost-model adds the cost of each exit condition for multi-exit loops, while in VPlan there's a single condition to control the loop. To match the legacy cost initially, pre-compute the cost for all exit conditions.

https://github.com/llvm/llvm-project/pull/92555