[FIRRTL] Optimize analog-connect-lowering

[tymcauley]

In #4466, I introduced a workaround to resolve #4112, which removes some unnecessary alias/assign code when connecting module instances with analog wires. However, as noted in the PR comments, the fix isn't quite ideal since it doesn't avoid generating some wires. Those wires get removed by DCE, but ideally they wouldn't exist in the first place.

If those wires were removed, then we could make the test code introduced in that same PR a bit nicer: it could be written as all CHECK-NEXT statements, since those temporary wires wouldn't be polluting the lower-firrtl-to-hw output.

[uenoku]

Maybe we can port the optimizations implemented in LowerToHW to normal canonicalizers?