This change helps in consolidating various methods to take `Referenceable*` as parameter instead
of child classes `Interval*` or `RegRecord*`. With that, it will be possible to have a common
logic in those methods that will check how many registers to process based upon either the information
of `regCount` in `Interval` class or `regCount` in `RegRecord` class.

Also splitted updateAssignedInterval() in `updateAssignedInterval()` that operates when `interval != nullptr`
and `clearAssignedInterval()`.

…te in batch.

At some places, we do things at individual register level. They used to take `RegRecord*`
and we would operate on the register. They will never have type other than `TYP_INT` / `TYP_FLOAT`.
However, the `Interval` can have a `registerType` that represents the type of the variable it
represents. So we separate the methods that operate on registers in the ones that does things
on single register (and hence takes `RegRecord*`) and the ones that can operate on more than
one registers (and hence takes `Interval*`).

Also add few methods that just updates the updateNextIntervalRef(), updateSpillCost()
as well as methods to get the firstRegister() of the series.

In earlier commits, I consolidated these 2 methods taking `Referenceable*` but proper
way to do is split in overloaded method. One set takes `Interval*` and operates on the
type of `Interval*` while the other set operates at individual `register` level.

…terval()