transforms.formal: ensure named statements as output

[ekiwi]

This fixes a problem where an assume in a submodule would loose its name and thus treadle would fail.

Contributor Checklist

• Did you add at least one test demonstrating the PR?
• 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?

Type of Improvement

• bug fix

API Impact

• just makes sure that the output of these passes does not violate EnsureNamedStatements

Backend Code Generation Impact

• none, but the bug was messing with firrtl downstream consumers like treadle

Desired Merge Strategy

• squash

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?
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• Did you mark as Please Merge?

[chick]

lgtm