Inconsistencies Around Invalidation

Invalidate is currently being emitted kind of strangely and inconsistently.

First, for `Analog` types, an invalidation is emitted. However, this invalidation has no effect.

Consider:

``` scala
//> using scala "2.13.11"
//> using repository sonatype-s01:snapshots
//> using lib "org.chipsalliance::chisel::6.0.0-M3"
//> using plugin "org.chipsalliance:::chisel-plugin::6.0.0-M3"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
import chisel3.experimental.Analog
import circt.stage.ChiselStage

class Bar extends Bundle {
val a = Bool()
val b = Analog(1.W)
}

class Foo extends RawModule {
val out = IO(new Bar)

out :<= DontCare
}

object Main extends App {
println(
ChiselStage.emitCHIRRTL(
gen = new Foo
)
)
}
```

This emits:

```
FIRRTL version 3.1.0
circuit Foo :
module Foo :
output out : { a : UInt<1>, b : Analog<1>}

invalidate out.b
invalidate out.a
```

The `invalidate out.b` is a no-op.

Second, a connect to `DontCare` doesn't properly exclude probes:

``` scala
//> using scala "2.13.11"
//> using repository sonatype-s01:snapshots
//> using lib "org.chipsalliance::chisel::6.0.0-M3"
//> using plugin "org.chipsalliance:::chisel-plugin::6.0.0-M3"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
import chisel3.probe.Probe
import circt.stage.ChiselStage

class Bar extends Bundle {
val a = Bool()
val b = Probe(Bool())
}

class Foo extends RawModule {
val out = IO(new Bar)

out.excludeProbes :<= DontCare
}

object Main extends App {
println(
ChiselStage.emitCHIRRTL(
gen = new Foo
)
)
}
```

Current error:

```
[error] ProbeInvalid.scala 19:21: excluded field Foo.out.b: IO[Bool] has matching non-excluded field DontCare()
[error] There were 1 error(s) during hardware elaboration.
Exception in thread "main" chisel3.internal.Errors: Fatal errors during hardware elaboration. Look above for error list. Rerun with --throw-on-first-error if you wish to see a stack trace.
```

What I'm leaning towards is Chisel's `Connectable` should either be directly cognizant of only operating on FIRRTL Connectable types (definition: https://github.com/chipsalliance/firrtl-spec/blob/dcd63187d99a923b67caec597e374b8e400f070b/spec.md#connectable-types) or there should be helpers that can be used to only work with the FIRRTL Connectable parts of a `Data`. Put differently, it seems like the verbose method below should exist:

```
foo.excludeUnconnectable :<= DontCare
```

Alternatively, all the Chisel `Connectable` operators should only be operating on types which are, in fact FIRRTL Connectable.

#### Use Cases

1. Invalidating a wire that has probes in it without having to use `DataMirror` to exclude probes.
2. Tighten restrictions on what is illegal FIRRTL by defining invalidation only on FIRRTL Connectable types.