[silgen] Refactor emitShuffle (~300 loc) into smaller methods (< 90 loc) on TupleShuffleArgEmitter.

This should be a pure NFC refactor. I just moved function scope state into the
helper struct and split up the already nicely scoped parts of the function into
separate methods if they were large.

In future commits, there are a bunch of improvements that can be made by
reducing indentation.

Author: gottesmm