JIT: stateful local ref counts and weights (#19068)

JIT: stateful local ref counts and weights (#19068)

Introduce a notion of state for local var ref counts and weighted ref counts.
Accesses and current state must agree.

State is invalid initially, enabled for an early period around bits of morph,
invalid again for a time, and then enabled normally once lvaMarkRefs is called.

Accesses normally specify RCS_NORMAL as the desired state, but in the accesses
of selected ref counts in morph, specify RCS_EARLY.

Revise how we decide if normal ref counting is active by changing
`lvaLocalVarRefCounted` into a method.

Update `gtIsLikelyRegVar` to not access ref counts when they're not in a valid
state.

Change weight APIs over to use `weight_t`.