Enrich ShiftRegister with SyncReadMem-based implementation. (backport #2891)

[mergify]

This is an automatic backport of pull request #2891 done by Mergify.

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

---

Original PR Body

Contributor Checklist

• 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?
• Did you add appropriate documentation in docs/src?
• 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

• backend code generation
• new feature/API
• performance improvement

API Impact

This feature extends the current API preserving full backward compatibility.

Backend Code Generation Impact

If SyncReadMem-based Shift Register implementation is selected this will impact the generated Verilog code.

Desired Merge Strategy

No preference.

Release Notes

Supplement ShiftRegister with SyncReadMem-based implementation.

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?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
• Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
• Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.