[LowerFIRRTLToHW] Create less wires with instance ports

[youngar]

When we lower input ports on instances, if there are zero or multiple writers to the input port, we create a wire and use that as the operand to the new instance operation. There was a change to this code to use hw::WireOps, which have their input value as an operand. Since hw::WireOp's cannot have multiple writers, the code is now "the last writer wins". The wire's input is updated when processing connect-like operations - we will replace the previous writer to the wire.

We already have a helper in this code to gracefully handle this situation called createBackedge. A backedge is a placeholder for a value which will be determined later. We can use a backedge as an operand to the lowered instance op, and when we find the connect operation driving a value to the old instance port, replace the backedge with the driven value.

The backedge helper in this pass gracefully handles undriven backedges and combinational loops of backedges; if a backedge has not been replaced by the end of the module body, it will be replaced with an undriven wire.

An assert had to be removed which prevented a backedge from being updated multiple times. Regular connect ops were updated to support backedges, instead of just strict connects.

This change is NFC in the FIRRTL pipeline, where the corner cases of combinational loops and undriven ports are illegal. This better supports the two illegal cases of undriven inputs and combinational loops. In the future we should replace all usages of createTmpHWWireOp with the backedge helpers.

[fabianschuiki]

LGTM! I love how this gets rid of another batch of temporary wires 🎉

[dtzSiFive]

Amazing improvement, LGTM!