[FIRRTL] zero-width aggregate consumers crash or produce invalid IR

CIRCT mishandles legal consumers of zero-width aggregate values.

The following legal FIRRTL code crashes firtool:

FIRRTL version 4.0.0
circuit Top:
  module Child:
    output out: {nested: {zero: UInt<1>[0]}}
  public module Top:
    output size: UInt<32>
    inst child of Child
    node sizeNode = intrinsic(circt_sizeof : UInt<32>, child.out)
    connect size, sizeNode

Run with:

./bin/firtool --scalarize-internal-modules=true ./test.fir

LowerSignatures removes the child instance result because the aggregate has no fields. Its cleanup only handles connects and aggregate projections, so the intrinsic is treated as an unexpected user and the compiler aborts:

unexpected user of zero-width instance result
UNREACHABLE executed at lib/Dialect/FIRRTL/Transforms/LowerSignatures.cpp:419!

Relevant backtrace:

#7  eraseZeroWidthUsers(...)                         LowerSignatures.cpp:0:0
#8  lowerModuleBody(...)                             LowerSignatures.cpp:0:0
#9  mlir::detail::walk<mlir::ReverseIterator>(...)
#10 LowerSignaturesPass::runOnOperation(...)

Start in lib/Dialect/FIRRTL/Transforms/LowerSignatures.cpp, especially eraseZeroWidthUsers and lowerModuleBody, and reproduce the failure with the firtool command shown in the issue. Trace how the intrinsic user is handled when the instance result has no fields. Done means the legal FIRRTL example no longer crashes and does not produce invalid IR.