Option convenience ~refchecks for before and after phase

[som-snytt]

Scala 2 scalac -Vprint:~tailcalls as a convenience for printing before and after the named phase. You don't always remember what comes before without checking -Vphases, or maybe there is a plugin phase.

This might be higher payoff in Scala 3, where the innovative mini/mega phase construction is even more confusing.

Riffing a bit, it could also select the previous phase at which there was a difference. -Vprint:^tailrec

Avoiding -Vprint:erasure,tailrec. Well, that is an easy one. -VcrossVersionChecks,explicitOuter.