ExpandWhens: ensure that statement names are maintained

[ekiwi]

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

• bug fix

API Impact

• None

Backend Code Generation Impact

• No

Desired Merge Strategy

• squash

Release Notes

n/a

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?

[jackkoenig]

LGTM