[FIRRTL][LowerToHW] An uninstantiated module can crash LowerToHW

Nobody has claimed this yet.

I came across the following failure. The problem occurs if a module which is not under the main module in the instance graph includes reference type ports:

```mlir
firrtl.circuit "Foo" {
firrtl.module @Foo() {}
firrtl.module @Bar(in %a: !firrtl.uint<1>, out %b: !firrtl.probe>) {
%0 = firrtl.ref.send %a : !firrtl.uint<1>
firrtl.ref.define %b, %0 : !firrtl.probe>
}
}
```

The failure trips an assert in `LowerToHW` [here](https://github.com/llvm/circt/blob/main/lib/Conversion/FIRRTLToHW/LowerToHW.cpp#L2337):

```
Assertion failed: ((!result || !isa(result.getType())) && "Lowering didn't turn a FIRRTL value into a non-FIRRTL value"), function setLowering, file LowerToHW.cpp, line 2338.
```

Start with the FIRRTL reproducer in the issue and inspect LowerToHW.cpp around line 2337, where setLowering asserts on the uninstantiated module's reference-typed values. Run the reproducer to confirm the crash, then trace how the uninstantiated module and its reference type ports are handled; done means LowerToHW processes this input without triggering the assertion.

Written by the indexing model from the issue text.