JIT: refactor ref count computation into a reusable utility method (#19240)

JIT: refactor ref count computation into a reusable utility method (#19240)

Extract out the basic ref count computation into a method that we
can conceptually call later on if we want to recompute counts.

Move one existing RCS_EARLY count for promoted fields of register
args into this recomputation since losing this count bump causes
quite a few diffs.

The hope is to eventually call this method again later in the jit
phase pipeline and then possibly get rid of all the (non-early)
incremental count maintenance we do currently.

Part of #18969