fix[codegen]: relax the filter for augassign oob check

[charles-cooper]

What I did

relax the filter in #4487

How I did it

How to verify it

Commit message

this commit relaxes the filter introduced in dd5a3d9e0f1e8685. the
filter was valid, but blocked too many user programs. this commit
updates the filter so that the external call check is only applied if
there is a state variable on the lhs. it also updates the external call
check to only consider state-modifying calls, since staticcalls cannot
modify the lhs.

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.07%. Comparing base (1339177) to head (be05638).
Report is 97 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4497   +/-   ##
=======================================
  Coverage   92.06%   92.07%           
=======================================
  Files         120      120           
  Lines       17335    17343    +8     
  Branches     2935     2937    +2     
=======================================
+ Hits        15960    15968    +8     
  Misses        957      957           
  Partials      418      418           

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[charles-cooper]

note: immutables count as state variables