[VPlan] Split out VPBlendRecipe simplifications from simplifyRecipes. NFC (#134073)
This is split off from #133977
VPBlendRecipe normalisation is sensitive to the number of users a mask
has, so should probably be run after the masks are simplified as much as
possible.
Note this could be run after removeDeadRecipes but this causes test
diffs, some regressions, so this is left to a later patch.