Create explicit class in the IR for initializer expressions. NFC

[sbc100]

Rather than treating an init expression as a raw expressions list wrap it into its own class. While this does nothing now I plan to followup by attaching an end_loc (like we do for Block) so that we can have the validator handle checking the correct placement of the END instruction.

Its also good for readability to have an explict class for this class of expression list.