OptimizeInstructions: Optimize subsequent struct.sets after struct.new_with_default

[kripken]

Before we preferred not to add default values, as that increases code size. But since
#6495 we turn more things into struct.new_with default, so it is important to handle
this. It seems likely that in most cases the code size downside of adding default
values is offset by avoiding a local.set later, so always do this (rather than add some
kind of heuristic).