add some small LLL optimizations

[charles-cooper]

What I did

Add a couple peephole assembly optimizations and an LLL optimization for truthy sequences. Code size for three large-ish contracts comes down about 2.5%.

How I did it

How to verify it

Use the vyper-lll tool to inspect the output of this LLL

(seq
 (assert (eq 1 2))
 (assert (ne 1 2))
 (if (ne 1 2) (revert 0 0))
 )

Description for the changelog

Optimize truthy sequences and peephole stack operations.

Cute Animal Picture

[fubuloubu]

Add a little comment explaining why the optimization works

[charles-cooper]

@fubuloubu see d73f6ee

[codecov-commenter]

Codecov Report

Merging #2493 (e27c938) into master (93a9579) will decrease coverage by 1.15%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #2493      +/-   ##
==========================================
- Coverage   84.60%   83.44%   -1.16%     
==========================================
  Files          93       93              
  Lines        9228     9133      -95     
  Branches     2086     2087       +1     
==========================================
- Hits         7807     7621     -186     
- Misses        935     1023      +88     
- Partials      486      489       +3     

Impacted Files	Coverage Δ
vyper/cli/vyper_lll.py	0.00% <0.00%> (ø)
vyper/lll/compile_lll.py	94.48% <100.00%> (+0.13%)	⬆️
vyper/lll/optimizer.py	84.07% <100.00%> (ø)
vyper/ast/annotation.py	87.87% <0.00%> (-6.82%)	⬇️
vyper/semantics/types/value/numeric.py	80.21% <0.00%> (-5.81%)	⬇️
vyper/builtin_functions/functions.py	83.72% <0.00%> (-5.57%)	⬇️
vyper/old_codegen/expr.py	74.02% <0.00%> (-5.08%)	⬇️
vyper/ast/validation.py	68.88% <0.00%> (-4.45%)	⬇️
vyper/ovm/transpile_lll.py	61.33% <0.00%> (-2.65%)	⬇️
vyper/ast/nodes.py	92.06% <0.00%> (-1.53%)	⬇️
... and 15 more

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