[LV] Enforce order of reductions with intermediate stores in VPlan (NFC)

Reductions with intermediate stores currently need to be fixed in order
of their intermediate stores. Instead of doing this at fixup time after
code has been generated, sort the reductions in adjustRecipesForReductions.

This makes the order explicit in VPlan and will enable removing
fixReductions with modeling computing the final reduction result in
VPlan, followed by also modeling the intermediate stores explicitly.