feat: optimize dload/mstore sequences

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

merge_dload was defined in 5dc3ac7, but was not applied.

this commit also adds a rewrite rule for single dload/mstore patterns,
any `(mstore dst (dload src))` (which compiles to a `codecopy` followed
by an `mload` and `mstore`) is rewritten to a `dloadbytes` (which
compiles directly to a `codecopy`).

this rule saves 4 bytes / ~10 gas per rewrite. for instance, it shaves
25 bytes off `VaultV3.vy`, 50 bytes off `CurveTricryptoOptimizedWETH.vy`
and 75 bytes off `CurveStableSwapMetaNG.vy` (basically 0.1%-0.3%).

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #3525 (2082fcf) into master (9e3b9a2) will increase coverage by 0.02%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3525      +/-   ##
==========================================
+ Coverage   89.16%   89.18%   +0.02%     
==========================================
  Files          84       84              
  Lines       10953    10970      +17     
  Branches     2503     2506       +3     
==========================================
+ Hits         9766     9784      +18     
+ Misses        775      774       -1     
  Partials      412      412              

Impacted Files	Coverage Δ
vyper/ir/optimizer.py	98.30% <100.00%> (+0.35%)	⬆️

... and 1 file with indirect coverage changes

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