fix: edge cases in safepow

[charles-cooper]

What I did

fix #2984, #2986, #2987, #2989

How I did it

How to verify it

Commit message

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Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2983 (bc8b00b) into master (be2c59a) will increase coverage by 0.10%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #2983      +/-   ##
==========================================
+ Coverage   88.01%   88.11%   +0.10%     
==========================================
  Files          97       97              
  Lines       10868    10886      +18     
  Branches     2572     2578       +6     
==========================================
+ Hits         9565     9592      +27     
+ Misses        844      838       -6     
+ Partials      459      456       -3     

Impacted Files	Coverage Δ
vyper/codegen/arithmetic.py	77.01% <89.47%> (+1.25%)	⬆️
vyper/codegen/core.py	84.99% <0.00%> (+0.08%)	⬆️
vyper/ir/optimizer.py	97.86% <0.00%> (+2.83%)	⬆️

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