[firtool] Run canonicalization before LowerLayers

[seldridge]

Run canonicalization before LowerLayers in order to optimize things which are difficult to do after layerblocks have been converted to modules. The conversion to modules means that many trivial canonicalizers are now very difficult to do as they require inter-module analysis.

Ideally, it would be best if LowerLayers ran as late as possible. (Generally any outlining should run as late as possible.) However, we aren't quite ready to make this harder move of pushing LowerLayers to the end of the FIRRTL pipeline.

Fixes #7533.

[seldridge]

Discussed offline with @dtzSiFive. There appears to be at least one register canonicalizer (see foldHiddenReset in FIRRTLFolds.cpp) and this shouldn't be run before RandomizeRegisterInit because that can cause changes in the randomization behavior due to optimizations.

It would be better to move LowerLayers later in the pipeline so that it runs after canonicalization. This would require landing #7398 to move it after ModuleInliner and see about moving it after RandomizeRegisterInit. The latter change would be beneficial as this would ensure randomization stability involving specialization that enables layers (though not guarantee it for disabling layers).

[uenoku]

IIRC we have wanted to perform register optimization only in `RegisterOptimization pass so removing foldHiddenReset from canonicalizer looks fine solution to me.