refactor[venom]: add make_nop() helper function

[harkal]

What I did

How I did it

How to verify it

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.11%. Comparing base (1b0fa5b) to head (ebead66).
Report is 95 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/basicblock.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4470      +/-   ##
==========================================
+ Coverage   92.09%   92.11%   +0.01%     
==========================================
  Files         119      119              
  Lines       16931    16933       +2     
  Branches     2865     2865              
==========================================
+ Hits        15593    15597       +4     
+ Misses        919      917       -2     
  Partials      419      419              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.