[LoopVectorize] Change PredicatedBBsAfterVectorization to be per VF

When calculating the cost of Instruction::Br in getInstructionCost
we query PredicatedBBsAfterVectorization to see if there is a
scalar predicated block. However, this meant that the decisions
being made for a given fixed-width VF were affecting the cost for a
scalable VF. As a result we were returning InstructionCost::Invalid
pointlessly for a scalable VF that should have a low cost. I
encountered this for some loops when enabling tail-folding for
scalable VFs.

Test added here:

  Transforms/LoopVectorize/AArch64/sve-tail-folding-cost.ll

Differential Revision: https://reviews.llvm.org/D128272