[VPlan] Compute NumPredStores for VPReplicateRecipe costs in VPlan.

Compute the number of predicated stores directly in VPlan instead of
using CM.useEmulatedMaskMemRefHack(), which will only account for the
number of predicated stores for the last VF the legacy cost model
considered.

Fixes https://github.com/llvm/llvm-project/issues/181183

GitOrigin-RevId: b3dcf485d23a574e4c377739a32468240a87c2fc