Micro-optimize String building in _computeName

[jackkoenig]

This is very much a micro-optimization, but I was looking at Flamegraphs and noticed 2 StringBuilders when there should only be 1. The String concatenation before adding to the String builder results in an unnecessary String copy. This is peanuts compared to most of the object creation going on in Chisel, but it is noticeable in a Flamegraph so I figured I'd fix it.

Contributor Checklist

• [NA] Did you add Scaladoc to every public function/method?
• 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?
• [NA] Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
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• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• performance improvement

API Impact

No impact

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Micro-optimize performance of computing names of signals

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• Did you review?
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• Did you mark as Please Merge?