Assertion/revert optimizations

[iamdefinitelyahuman]

What I did

LLL-level optimizations around assertions and reverts.

How I did it

• check the assertion before storing the error string in memory
• use revert macro instead of assert where possible
• avoid assert check if test statement is a literal

How to verify it

Run tests.

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[codecov-io]

Codecov Report

Merging #2198 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2198      +/-   ##
==========================================
- Coverage   85.30%   85.26%   -0.04%     
==========================================
  Files          83       83              
  Lines        8524     8523       -1     
  Branches     2061     2063       +2     
==========================================
- Hits         7271     7267       -4     
- Misses        747      748       +1     
- Partials      506      508       +2     

Impacted Files	Coverage Δ
vyper/compile_lll.py	92.55% <ø> (-1.01%)	⬇️
vyper/opcodes.py	100.00% <ø> (ø)
vyper/optimizer.py	83.07% <100.00%> (-0.26%)	⬇️
vyper/parser/stmt.py	80.76% <100.00%> (+0.48%)	⬆️

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