Added a visited cycle-detection guard in the *ssa.Phi case

[ccojocar]

fixes #1624

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.07%. Comparing base (48e8716) to head (ac39e53).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
taint/taint.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1626      +/-   ##
==========================================
- Coverage   81.09%   81.07%   -0.03%     
==========================================
  Files         108      108              
  Lines       10093    10096       +3     
==========================================
  Hits         8185     8185              
- Misses       1423     1426       +3     
  Partials      485      485              

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