Recommit "[VPlan] Add recipe for first-order rec phis, make splicing explicit."

This reverts the revert commit b1777b04.

The patch originally got reverted due to a crash:
https://bugs.chromium.org/p/chromium/issues/detail?id=1232798#c2

The underlying issue was that we were not using the stored values from
the modified memory recipes, but the out-of-date values directly from
the IR (accessed via the VPlan). This should be fixed in d995d637. A
reduced version of the reproducer has been added in 93664503.