compiler: Fix MPI with CondDim + multiple loc_indices

[FabioLuporini]

Flagged as bug-C-minor because it only pops up in a fairly contrived situation and not flagged by any users...

But still a bug

[codecov[bot]]

Codecov Report

:x: Patch coverage is 66.12903% with 21 lines in your changes missing coverage. Please review. :warning: Please upload report for BASE (main@cfc5a08). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
tests/test_mpi.py	16.66%	10 Missing :warning:
devito/ir/clusters/algorithms.py	79.54%	8 Missing and 1 partial :warning:
devito/passes/iet/mpi.py	0.00%	0 Missing and 2 partials :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2696   +/-   ##
=======================================
  Coverage        ?   78.30%           
=======================================
  Files           ?      248           
  Lines           ?    49678           
  Branches        ?     4376           
=======================================
  Hits            ?    38902           
  Misses          ?     9958           
  Partials        ?      818           

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