JIT: rework phase objects so we can use them more widely (#31808)

JIT: rework phase objects so we can use them more widely (#31808)

Next part of #2109.

Add support for phases that are simply compiler methods or lambdas. These are
not used yet -- the plan is to introduce new phases gradually, cleaning up
redundant checking and dumping along the way. This will happen in subsequent
changes.

Remove a bit of now-redundant post-phase dumping and checking from lower.

Add the active phase to the assertion text, so we have some context.