Structs are split to remove builtin members to create valid SPIR-V.  In this
process, an outer structure array dimension may be propegated onto the
now-removed builtin variables.  For example, a mystruct[3].position ->
position[3].  The copy between the split and unsplit forms would handle
this in some cases, but not if the array dimension was at different levels
of aggregate.

It now does this, but may not handle arbitrary composite types.  Unclear if
that has any semantic meaning for builtins though.