feat[venom]: add load elimination

[charles-cooper]

What I did

add primitive sload/tload/mload elimination

How I did it

very primitive, keeps latest mload or mstore in a one-element "lattice", and weakens the sload/mload/tload to a store instruction if there is a hit on the same key.

How to verify it

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 12.28070% with 50 lines in your changes missing coverage. Please review.

Project coverage is 51.22%. Comparing base (b3ea663) to head (6923199).

Files with missing lines	Patch %	Lines
vyper/venom/passes/load_elimination.py	10.90%	49 Missing ⚠️
vyper/venom/__init__.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (b3ea663) and HEAD (6923199). Click for more details.

HEAD has 125 uploads less than BASE

Flag	BASE (b3ea663)	HEAD (6923199)
	138	13

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4265       +/-   ##
===========================================
- Coverage   91.40%   51.22%   -40.18%     
===========================================
  Files         112      113        +1     
  Lines       15927    15984       +57     
  Branches     2694     2711       +17     
===========================================
- Hits        14558     8188     -6370     
- Misses        935     7166     +6231     
- Partials      434      630      +196     

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