Implement NoCommonSubexpressionElimination

[sequencer]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you update the FIRRTL spec to include every new feature/behavior?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

Add --no-common-subexpression-elimination to disable CSE.

Backend Code Generation Impact

None

Desired Merge Strategy

• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Release Notes

Add --no-common-subexpression-elimination to disable CSE.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?

[sequencer]

Asking for review from @seldridge.

[seldridge]

Looks great. It makes sense to me to have more fine-grained control over which optimizations are run.

[seldridge]

I marked this as 1.5 due to perceived binary incompatibility issues around the change to the inheritance/mix-ins for CSE. (I think this is the case?) This patch be backported with a different approach, though I'm not sure this is necessary to backport. @sequencer: if you need this in 1.4.x or earlier, feel free to update. 😄

[sequencer]

@seldridge 1.5 is fine :) let's put it into 1.5.

[lingscale]

Just like NoDCEAnnotation and NoConstantPropagationAnnotation, maybe NoCommonSubexpressionEliminationAnnotation more perfect than NoCommonSubexpressionElimination?