[firrtl] Move LowerLayers after LowerXMR

[seldridge]

Move the LowerLayers pass after the LowerXMR pass. To do this, all
passes at the end of the CHIRRTL to Low FIRRTL pipeline are moved after
LowerXMR. This is necessary because the LowerLayers pass cannot, at
present, be moved after the passes at the end of the pipeline.

This is done to enable forcing out of layers. By lowering probes to XMRs,
the layers can be lowered trivially to modules/instances and their XMRs
will now (seemingly) magically just work. By doing the loweirng in this
way, it avoids ever having to represent an input probe in the FIRRTL
dialect.

A consequence of this is that there are now simplifying
assumptions (preconditions) that can be made about the LowerLayers pass:

1. It will never see a number of probe ops because the LowerXMR pass has
   a postcondition that all of these are removed.

2. Input and output ports can never be created on modules created from
   layer blocks.

While I am generally always in favor of passes being relocatable anywhere
in the pipeline, this is one pass that really does not make sense to be
relocatable. In effect, this pass is part of the lowering from FIRRTL to
HW. There is no point in being able to use it earlier in the pipeline.
That said, it still is tested to work with things which we may one-day
preserve, like WhenOps.

Todo

• Handle zero width

[seldridge]

The changes in this PR introduce a lot of unreachable code in LowerLayers. I will clean this up as long as there is consensus that we want to enforce the precondition that LowerXMR runs before this always (and thereby certain ops will never be seen). It is possible to continue to support this, but it will not be load bearing in firtool.