microsoft/compiler: Use lowered io for non-explicit io

Looking through the set of piglit fails, one root cause is inter-stage I/O with structs. The deref-based I/O handling we had would be fragile to handle this, where with lowered I/O, the accesses inside of the shader would become free. It also really, really simplifies the handling of things like compact arrays and GS (per-vertex) inputs - see overall series diff.

No change in piglit results yet since this doesn't actually support structs yet - the variable -> DXIL signature parts need to be updated to emit the correct varyings first.