Optimization: division safemath when divisor is a literal

[iamdefinitelyahuman]

What I did

Remove the safemath check when dividing by a literal value.

How I did it

If the divisor is a literal, remove the clamp_nonzero instruction within the LLL.

How to verify it

Run tests.

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[codecov-commenter]

Codecov Report

Merging #2173 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2173      +/-   ##
==========================================
+ Coverage   85.22%   85.23%   +0.01%     
==========================================
  Files          83       83              
  Lines        8330     8336       +6     
  Branches     2009     2011       +2     
==========================================
+ Hits         7099     7105       +6     
  Misses        731      731              
  Partials      500      500              

Impacted Files	Coverage Δ
vyper/parser/expr.py	77.04% <100.00%> (+0.28%)	⬆️

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