fixup! fix exprt::opX accesses in linker_script_merge

[tautschnig]

In one case 7bf94f1 wrongly turned index_exprt::op0 into index_exprt::index() when that should be ::array().

Running tests on aarch64 was thus broken (but we don't yet do that in CI).

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[codecov]

Codecov Report

Attention: Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.

Project coverage is 80.36%. Comparing base (c9653bd) to head (2e054be).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/goto-cc/linker_script_merge.cpp	64.70%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8657      +/-   ##
===========================================
- Coverage    80.36%   80.36%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206981   206981              
  Branches        73       73              
===========================================
- Hits        166340   166337       -3     
- Misses       40641    40644       +3     

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