[LowerClasses] Lower classes that instantiate properties.

[mikeurbach]

The previous logic would create classes for any FIRRTL modules that had property ports. However, if some intermediate modules had property ports, but their parents did not, we would not convert their parents, which would break pass invariants and lead to failed assertions.

This adds logic to also convert any FIRRTL modules that instantiate FIRRTL modules that will be converted. This upholds the pass invariants, and makes it robust in case property ports are used in part of the hierarchy but not exported from the top-level module of the circuit.

[rwy7]

Note to self: this is safe because: for ops like extmodule or intmodule, which have no body, we still create a region, it's just that the region has no blocks.

Also note to self, this will catch firrtl.objects ops as well, including when a firrtl.object has no ports (because the object op has a single result, which is a property).

[rwy7]

LGTM. Thoughts for another PR, maybe:

1: Lower-Classes will still leave some property ops behind in the hardware, for example this is left unchanged:

firrtl.circuit "Foo" {
  firrtl.module private @Foo() {
    %c0 = firrtl.integer 0
  }
}

2: Lower-Classes is now creating unnecessary classes, which (IIRC) symbolDCE can't remove.

[mikeurbach]

Thanks for taking a look.

1. is just dead code, so i'm not too concerned about that. any canonicalizer or cse should remove that. if the property value %0 was used in an instance somewhere, Foo would get converted to a class with this change
2. i would argue that these are necessary classes. in the test case example, SubmoduleWithProperty definitely needs to be lowered to a class. where should the instance of that class be instantiated? it seems to me that we have to convert ModuleWithPropertySubmodule to a class and instantiate SubmoduleWithProperty there. ModuleWithPropertySubmodule is useless in the sense that you can't actually get at the instance of SubmoduleWithProperty from this IR example, but that isn't illegal, and if we don't make the instance somewhere the pass crashes with assertions.