dialects: (scf) make iteration variable type explicit

[alexarice]

In mlir, if the iteration variable of scf.for does not have an explicit type, then it is assumed to be index. This change matches this behaviour in xDSL.

[webmiche]

I see that you touched some csl stuff. Does this affect anything there? (@dk949, @n-io)

Otherwise, LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.97%. Comparing base (d95e30d) to head (1c0313a).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3138      +/-   ##
==========================================
+ Coverage   89.94%   89.97%   +0.02%     
==========================================
  Files         425      426       +1     
  Lines       53571    53711     +140     
  Branches     8299     8319      +20     
==========================================
+ Hits        48185    48325     +140     
+ Misses       4044     4043       -1     
- Partials     1342     1343       +1     

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[dk949]

I see that you touched some csl stuff. Does this affect anything there? (@dk949, @n-io)

Otherwise, LGTM!

We use i16s for loop indices which used to be inferred from ub, lb and step, but I'm guessing no longer is.

So, no, I don't think this affects anything.