Fix conversion overflow false positives

[czechbol]

This pull request hopefully fixes the issue of false positives in the overflow analysis whenever bounds checks are performed or the size is already pre-determined.

Disclaimer:
This is my first time contributing to any linter and I don't consider myself being an AST wizard so please be more cautious when reviewing this code.

fixes #1187

[ccojocar]

Thank you for this great contribution!

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.

Project coverage is 67.34%. Comparing base (ab3f6c1) to head (df0206f).
Report is 3 commits behind head on master.

Files	Patch %	Lines
analyzers/conversion_overflow.go	77.35%	7 Missing and 5 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
+ Coverage   67.30%   67.34%   +0.03%     
==========================================
  Files          74       74              
  Lines        3992     4045      +53     
==========================================
+ Hits         2687     2724      +37     
- Misses       1186     1195       +9     
- Partials      119      126       +7     

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[ccojocar]

@czechbol It seems that there some more use cases to handle #1187 (comment). I would be great if you could also check the bounds. Thanks