[Bug] Peephole optimization could create unused locals which should be cleaned up

[vineethk]

🐛 Bug

When peephole optimization runs, it can end up deleting instructions. If StLoc instructions are deleted, the locals that they used to refer to may no longer have references in the code, but we may still retain the original slots for those unused locals.

We should do a pass over the generated code (after peephole optimization), and if any local does not have a corresponding (one or more) StLoc instruction, then the local should be eliminated.