Improve unboxing of strict fields

Improve unboxing of strict fields

Note [Recursive unboxing]
~~~~~~~~~~~~~~~~~~~~~~~~~
Be careful not to try to unbox this!
	data T = MkT !T Int
But it's the *argument* type that matters. This is fine:
	data S = MkS S !Int
because Int is non-recursive.

Before this patch, we were only doing the unboxing if the *parent*
data type was non-recursive (eg that meant S was not unboxed), but
that is over-conservative.

This showed up with indexed data types (thanks to Roman for finding it)
because indexed data types are conservatively regarded as always recursive.