[FIRRTL][Inliner][InstChoice] Basic example doesn't work

[dtzSiFive]

Input:

FIRRTL version 4.0.0
circuit Foo:
  option Platform:
    FPGA
    ASIC

  module DefaultTarget:
    input clock: Clock

  module FPGATarget:
    input clock: Clock

  module ASICTarget:
    input clock: Clock

  public module Foo:
    input clock: Clock

    instchoice inst of DefaultTarget, Platform :
      FPGA => FPGATarget
      ASIC => ASICTarget

    inst.clock <= clock

This errors in the inliner, which deletes the target modules as "unused".

This is related to updating the InstanceGraph to understand instance choice (and our passes accordingly), but for this specific pass. Others may also need updating.