Optimize shift when 2nd arg is a literal

[iamdefinitelyahuman]

What I did

Optimize the LLL for shift when the distance being shifted is a literal.

How I did it

Avoid pushing to the stack when not necessary.

How to verify it

Run tests. I didn't add any - the behaviour is already well tested.

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[codecov-io]

Codecov Report

Merging #2201 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2201      +/-   ##
==========================================
+ Coverage   85.26%   85.27%   +0.01%     
==========================================
  Files          83       83              
  Lines        8523     8529       +6     
  Branches     2063     2065       +2     
==========================================
+ Hits         7267     7273       +6     
  Misses        748      748              
  Partials      508      508              

Impacted Files	Coverage Δ
vyper/functions/functions.py	89.41% <100.00%> (+0.08%)	⬆️

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