[llvm] Add IR2Vec as an observation space

[ChrisCummins]

This adds support for IR2Vec to CompilerGym's LLVM environments.

Credit to co-author @anilavakundu!

This PR supersedes #615. Fixes #449.

[codecov-commenter]

Codecov Report

Merging #560 (adce428) into development (c5b9289) will decrease coverage by 0.34%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #560      +/-   ##
===============================================
- Coverage        88.75%   88.41%   -0.35%     
===============================================
  Files              115      115              
  Lines             7019     7019              
===============================================
- Hits              6230     6206      -24     
- Misses             789      813      +24     

Impacted Files	Coverage Δ
compiler_gym/envs/compiler_env.py	88.39% <0.00%> (-3.96%)	⬇️
compiler_gym/views/observation.py	97.29% <0.00%> (-2.71%)	⬇️
compiler_gym/envs/llvm/datasets/cbench.py	79.42% <0.00%> (-1.09%)	⬇️
compiler_gym/service/connection.py	77.04% <0.00%> (-0.99%)	⬇️
compiler_gym/envs/gcc/service/gcc_service.py	96.70% <0.00%> (-0.37%)	⬇️
...loop_tool/service/loop_tool_compilation_session.py	90.47% <0.00%> (ø)

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