[VPlan] Process simplifyRecipes in a worklist
This brings simplifyRecipes further in line with InstCombine, and asides from unlocking more simplifications it also helps avoid spurious test churn whenever passes are moved around simplifyRecipes.
For now just push the new recipe onto the worklist, not its users.
This uses a post order traversal so we maintain the same simplification order as before.
I've gone through and checked every simplification we do is a canonicalisation that converges, and I checked on llvm-test-suite + SPEC CPU 2017 in various configurations that we don't hit any cycles.
